Yes, sure it is, if you can afford to shell out $1000 dollars for a playable gaming PC. :P

Not anymore, nvidia and amd have good entry level gpus for $100-$150

Putting 'demos' on other systems is a great idea if it leads to more sales of games on your own system.

They're little more than adverts for the main series and it seems to have paid off given Pokemon sales.

It may be simplistic, but its still a very succesfull business model when you have all those famous IPs already stablished in the market, which is why Sega did it, and which is why it'd make sense if Nintendo do it. Thats why people think they would/could do that. Because "it makes sense". Personally, I dont think they would, I think they would focus their efforts in something entirely different like mobile games, which is less risky and they're already joining that market. Having that said, as a Nintendo fan, if they were to fail in the console market I'd want them to become a third party developer, for the sole purpose of having them still develop games of their titular IPs.

If Nintendo goes third party then third parties will be have difficulties because they'll have to compete with Nintendo which is bad for third parties.

Eventually Sony/MS would only have 1st party and Nintendo titles and Nintendo's console would only have 3rd parties assuming Nintendo didn't make 1st party titles for their own system because they don't want to upset 3rd parties. If Nintendo made games for their own system then 3rd parties would be forced to compete with Nintendo on every system which is really bad for 3rd parties.

In time this would leave Nintendo as the only 3rd party developer because 3rd parties don't like to compete with Nintendo games which means Nintendo would have all the third parties.

If Nintendo ever stops developing consoles, then for sure they will go 3rd party

but when that will be, if ever, that is the question

PC is MS' platform.  They innovate with DirectX exactly to push gaming.  Their own PC games use that system.
(obviously there is competing APIs on PC, but MS is pushing their own there with their own products).
Sony in fact released alot of PC games, which is de facto on MS' turf.  
The arguments why releasing similar/identical games on a platform besides one's own console is a horrible idea
make just as little sense for these companies as for Nintendo.  MS releases PC/smartphone software on Apple.
Apple releases software on Windows/Android.  None of those companies even "failed" before doing so.

LOL

In many ways, yes... if you look at the meta scores for recent Nintendo games at least... and try to compare them to something like GTA V or BF1 .

They're not: Switch is coming next year.