Well, is like the last of us, GTAV, all other crossgen games that was on ps3 and then on ps4.

Face them as crossgen games.

Thats exactly what people disliked thé msot about this gen, making this gen less necessary,too much port, and it seems Nintendo is going full in with ports of games from last gen or cross gen games , based on rumors thzy could even break thé record of ports in à first year.

I don't think Nintendo are concerned about the sales of Mario Kart 8 in particular. However, they may be with some of their other titles like Bayonetta 2 and Xenoblade.

And yeah, to me it doesn't make a difference if the Switch Mario Kart game is 8.5 or 9. However, it does for Smash Bros. The lineup for Smash 4 is something I don't see them topping with the next instalment for me. If ever, due to the expensive liscenses they got for all those guest characters in particular. And I'm not confident that some of my favorites like Lucina will make a return when there may be a new Fire Emblem game out by then with new characters to promote.
So instead of throwing all of that away and start over from scratch, I think it would be better if they simply continued to build on it.

If we look at the console Mario Kart games, they come out around every 4,5 years on average. If we go by that, Mario Kart 9 could be expected around January 2019. A port can help them push the game out much earlier.
I also think Mario Kart and Smash are games that are particularly suited for an update approach, since as you said they're not for example story driven that warrant completely new entries. Look at Street Fighter 4 for example. Capcom upgraded the same game multiple times over the course of 7 years, and people played it to the very end.

Well, is like the last of us, GTAV, all other crossgen games that was on ps3 and then on ps4.

Face them as crossgen games.

Except The Last of Us is story driven and because of that it makes sense as to why 360 gamers that missed out on that game would want it when they purchase a PS4. Let's be honest, the same situation isn't true at all for Mario kart or Splatoon. People keep using these tired excuses of "But what about this game or what about xxx". I remember a time when people used Nintendo as the standard back in 2014 instead of the exception. All people did was criticize the PS4 for remasters. 

Well, 85 x360 million players missed tlou. also every non wii u owner missed splatoon and mk8. Much more peoplehandt access to ps3 remasters than wii U's. Also, this strategy work on ps4, now is a juggernault. Seems that what a few users in foruns contradict how market reacted(bougth remasters in millions, despite suposelly the bad pratice from sony). that what happened this time.  Also, the lots of developers that produced games from ps3 and ps4, are all wrong? All anual sport games, AC, CoD, lego, JD, tomb raider, metal gear, forza, etc. they are all wrong? Lets face, is just the ps4 version of a ps3 game. Sometimes, even games you could play on x360. Havent seen no one complaining, but is the same thing as GTAV and tlou: just a PS4 version of the game.

If anything, Playstation has more of an excuse for remasters than Nintendo does, which is ironic. 

Nintendo has the biggest 1st party presence of any console manufacturer, there's no denying that. If they released a Switch expecting to sell 7 million in the first year(that would be considered a great year for the Switch btw) then Zelda and mario ALONE would sell that. So why release these "cross platform" remasters lazily? There's only a few reasons : 

1. Nintendo doesn't trust the popularity of Zelda or Mario 

2. Nintendo doesn't have the 3rd party multiplayer support they  need

3. Nintendo doesn't have the 3rd party support needed for consistent Switch releases 

4. Nintendo feels multiplayer-centric titles is needed to sell the system

In all these instances if Nintendo doesn't cover these grounds(remember these are the only reasons for them to re-release these games as reasons to buy the system) than nintendo ABSOLUTELY deserves to fail. If they're insecure about their multiplayer 1st party line up? Absolutely deserve to fail for just releasing remasters. if  they don't have 3rd party support and consumers feel it is needed? Absolutely deserve to fail. I'm not going to excuse a company simply because they're releasing lazy ports. If the Switch NEEDS multiplayer 1st party titles for some odd reason, despite probably getting Mario, Zelda, and maybe Pikmin in the first year ... then Nintendo needs to get on it, not turn into Sony. 

Your considering something that is not always truth: Having ports doesnt mean they are not doing new games. Even in cases where nintnedo give other developer to make the port(TPHD), or when themselves start porting a game, they usually learn with the experience to how to develop a games for the system and the new games can be done faster. And also, they always have studios working in new games. And you forgot the main point: As switch is both console and handheld, Nintendo focused the production of handheld and console in a single device, so, is the double of standart stream of games.
IMHO, the more the games, the better. Even with ports, they are doing so.
Also, in ps4 case, having 2013 versions of ps3 games helped a lot the number of posssible options of games, so, it was good too. And we saw a ps2/wii level of sales.
Thats more evidence that the more games, the better.

your logic is awful

selling 7 million copies of a game on a system that has like 13 million owners is an INSANE attach rate, especially when considering that there haven't been THAT many Mario Kart system bundles

People have rated Mario Kart 8 as one of the best games in the series and even debated that it may be one of the best looking games this gen. Its not that Mario Kart Wii was so great that caused it to sell that many copies, its that the Wii install base was gigantic compared to the Wii U. Even so, proportionately Mario Kart 8 is selling at a wayyyy better attach rate than Mario Kart Wii did, which would suggest the fans prefer it

if another Nintendo system sells 100 million systems you'll see the next Mario Kart game have insane sales too

The reason the Wii U failed was because it was poor conceptually. I have NEVER heard someone claim that Nintendo's main IPs being mediocre was the issue with the Wii U. Mario 3D Land (albeit not my favorite) was rated solidly, Smash and Mario Kart rated fantastically, etc.

maybe a LACK of some of Nintendo's IPs hurt the Wii U (Zelda, Metroid, F Zero, a good Animal Crossing, a Pokemon home console game, etc. missing). 

the reality is though it was the concept of the system that failed. The majority of casual gamers and even a lot of old school Nintendo fans do not want a big tablet controller thing in their lap that they need to look down at. It raised the price of the system to quite high for what it was and didn't really add much to the experience 

Since I've never heard anything but praise for Nintendo's WiiU's games, and they've generally reviewed extremely well, I don't believe that's the concensus. The average consumer buys around 5 games for a system, so you can't ignore game output, or price of the system, being important factors. I think a big problem was that it didn't have enough good games for some, and that those games took a long time to come out for others. The first 3D Mario game came out a year after launch. Mario Kart took 1,5 years, and Smash came out almost 2 years after launch. And by that time people's interest can ware off and their money is instead spent elsewhere.
With your logic here, every gamer would buy a console, no matter what it costs, as long as it has one single good game.
Obviously things are not that simple. Quantity and price play an important role. Xbox One didn't make alterations to their game linup in their first quarter. They slashed the price by $100 by cutting loose dead weight that people didn't want to pay for (The Kinect). Nintendo had a similar problem with the expensive Gamepad, but they never did anything about it.

The reason the "casual Wii crowd" aren't buying WiiU isn't because of game quality. They're not interested in PS4 either. They're content with their free to play games on their mobile devices. Games like Dragonball Z Dokkan Battle have over 100 Million downloads. Nintendo managed to catch their interest with Wii's gimmic right before smartphones became a thing. That crowd is no longer a major factor in the console market.

So what Nintendo need to do is attract more of the core audience who are interested in their games. They have a potential audience for that on 3DS. It's no coincidence that Switch is portable.

Those games did not sell Gamecube. The consept of a second screen isn't bad (even if the price is hard to justify), what's bad, is that the screen is forced for some secondary function in almost all of the games - which is a software problem, not hardware.

Attach rate doesn't matter. Sales do. High attach rate only tells there are not many games to buy on a system.

People bought Wiis to play Mario Kart, not the other way around. Fans do not prefer Mario Kart 8, which is proven by its sales.

The casual gamers play largely what's popular at moment, it is the core audience of the industry. This is the people that play FIFA and COD.