If we were to disqualify consoles as proper Nintendo hardware, it would be fairer to say the Gamecube and Wii U are the two that don't count due to the fact that they are the least consistent with Nintendo's strategies of evolving toward intuitive and expanded interface options, and while also expanding multiplayer capabilities (particularly local).

Also, the SNES didn't mark a decline; first of all, for half its lifespan it was sold alongside the NES which was still getting strong support from Nintendo, while the SNES almost vanished in the months after the N64 launch. Second, the 16-bit generation had Nintendo's strongest years until the Wii/DS era. Third, the NES had a 10-11 year lifecycle while the SNES only had 5.

Dark Souls Trillogy isn't a remake of all three games, it's just all 3 Dark Souls games re-released in one package. You can have a problem with Bandai Namco's handling of the Switch release of Dark Souls Remastered, but don't act like this is a widespread problem for all Switch games. 

I'd say that indies are less dependent on "its new!" hype. Indies generally are retro graphics or just smaller games with a lot less hype, and obviously less marketing. AAA content are all about building hype and promoting the game to have a big release - all the excitement is around the release of the new game because its new, and when its not new anymore its old and old means no hype.

Indie games on the other hand usually already look old so from a buyer's perspective it doesn't really matter if its new or not, cuz it probably looks like it came out 25 years ago anyways. Probably a lot of times people don't even know if an indie game is new or not unless they really follow indie games. Heck I had no idea Celeste wasn't new when it came out on Switch, and didn't find out stardew valley wasn't new until after it came out on switch. But I sure as hell know, for example, that Skyrim is years old. Also I think people don't even really refer to indie games as ports when they come to other platforms. People aren't talking about indies like "oh its just an old port" the way they talk about large studio games "ah its an old port it better be majorly discounted and why are they being lazy and porting old games instead of making new games".

The perception of the indie game market is very different. Indie games sell well on Switch cuz nobody cares if they're old. And since they are indie, it often might be the only really popular game that person or indie studio has made, so they're not just gonna crank out another great seller from a stable of franchises that they don't have, their all star IS that one game. But people do care about when big studios just put old ports of games to make some quick cash instead of making new games.

It's not that the Switch doesn't play well with AAA content, it clearly does because Nintendo's AAA content is selling like crazy. It's just that third parties haven't put any new AAA content on the system. Yeah Skyrim sold a million, and that is fantastic because Skyrim hasn't been an exciting game to have on a system in YEARS! Put an actual brand new AAA third party game on Switch and it'll sell the same as AAA games always do.

Yeah that argument that Nintendo's systems are selling less and less could have been argued after the gamecube, and after the Wii U by saying the Wii was an anomaly. But now that the Switch will clearly sell better than any other Nintendo console except for maybe the Wii its a dead argument.

The decline from NES through Gamecube can be better understood by realizing that Nintendo went from having basically a monopoly on the market to competing in a crowded and competitive market in which they made some crucial missteps thinking they would always maintain control of the market. NES had no real competition, then Sega came along, then Sony came along as is really the only other company who has ever been able to compete toe-to-toe with Nintendo. After the gamecube Nintendo realized it couldn't compete for identical product space with two far larger companies because it doesn't have the resources of those companies. Sure N64 and Gamecube were definitely more powerful than PS1 and PS2, but got obliterated both times, and then you had Xbox squeezing Nintendo from the high end spec category so it wasn't competing on sales with less powerful much more third party friendly sony and with Microsoft in the game it no longer could even boast being the most advanced system. So with the Wii they made a huge change of business plan to let the other two fight over an identical product space (basically just letting Microsoft lose to Sony), while Nintendo forged a new path, breaking away from the "fewer sales every new generation" but relying more on doing something brand new and innovating which can be hit or miss. NES-SNES-N64-GC was the "no market to competitive market, release the most advanced tech possible" battle that they began losing once they no longer owned the industry, Wii-WiiU-Switch is the new strategy which has been hit-miss-hit so far. The pattern that Nintendo sells less every generation died more than a decade ago after the gamecube's run. The past 12 years Nintendo has been on a completely different sales pattern because they changed their business plan.