If they release their games on every platform then they need for hardware, but real Nintendo strength lies in that to be in hardware and software business.

They can have own hardware and games for it, they can have specific games for mobiles, they can have Amiibos and toys, theme parks, movies...and thats exactly what they doing now, Nintendo is expanding the business not shrinking by leaving hardware market. Saying that we will always have some kind of dedicated Nintendo hardware, Nintendo even with Wii U making profit.

Typical business failure. Long term profits should always be the goal not short term windfalls. Nintendo would slowly become a deluded brand as well as their cherished IPs if they where to whore them out everywhere.

I'd rather Nintendo continue to make good money and be around for another few decades than cash in, get filthy rich and then die in ten.

And if they didn't make the WiiU and releaesed all WiiU games on multiple platforms they would have made several times more profit.

No. Only crazy people like to play games on devices they already own, hence Nintendo would sell even fewer games than before.

Mobile is one thing, but I actually don't think that Nintendo titles would do very well on Sony and MS platforms, the demographics are too different, and it would also take away the incentive to purchase more than one console in many cases, meaning that perhaps some consumers would rather get the game on another platform instead on a Nintendo one, meaning one full hardware purchase in the hole and still only one piece of software sold.
Mobile; absolutely, they'll be going hard for this market, but PC and other console gamers wouldn't really be as a lucrative a market as many seem to think, in my humble opinion, but would rather cost them quite a few hardware sales, which are hard enough to come by for them these days.

MK8 was sold 7.5m buy 30. June.

Do you relly belive that MK8 woud sell tens of milions of copies on PS4/XB1!? Offcurse not, if some people relly want to play MK8 they would buy Wii U, probably combine sales of MK8 wouldnt pass 10m on PS4/XB1. And Nintendo make much more money from console like 1st party devolper for their consoles then they would make like 3rd party, 7.5m sold copies on Wii U made them more money than they would make with around 10m on XB1/PS4.

You missing point, they making profit even with their worst selling console in their history. You do realise that half of Nintendo revenue is from hardware business, and that actually biggest Nintendo strength is they have own softwar and hardware (own platform) and because of that basically they can make whatever they want!? 

By your logic, Sony would make more money if they would release Uncharted 4 (or any other game) on all other platforms, ask yourself why Sony dont do that!?

Because Sony get a very large amount of revenue from third party and ps+ subscriptions, until Nintendo get's third party support or some kind of subscription service having their own home console is irrelevant. Keeping 3DS exclusive is the right move because it does have some decent third party support and it has a fairly high installbase, and there really isn't any other platform viable for most 3ds games. Keeping the Wii exclusive was a good move, just because of it's insanely high installbase, MK Wii for example would not have sold anymore if it was released on all systems. Keeping the WiiU exclusive has held revenue back by a collossal amount.

Now keeping exclusivity is a good move still (even on the WiiU) Assuming that Nintendo's hardware returns to heights. If the NX gets substantial third party support and sales, a subscriptions service, or a very high installbase (60 million plus LTD) then it makes sense to maintain exclusivity. If that does not happen then Nintendo shouldn't have bothered.

Mario Kart Wii sold 35.8 million.

MK8 has sold 7.5 million.

All those people that enjoy playing Mario Kart didn't just dissapear off the face of the planet, they just don't want to buy a WiiU, yes I believe the game would sell 25 million+ on PS4/XBO/PC.

It's either that or the Mario Kart Brand is in HORRIFIC decline.

79% Less sales, and that's all due to the WiiU. Nintendo screwed up and gimped their software potentially MASSIVELY. Some truly phenomenal games on the system that have been held back by a completely unappealing system.

Alright let's not get too overboard here...

The only reason to keep software exclusive is draw people to your closed platform so you can make revenue from sources other than your own software. Third Party software, and subscription services. The WiiU has neither. It's a complete waste of a system and it's softwares potential.

We'll just have to wait and see if the NX is relevant as an exclusive platform.