Some games have a cap definitely, but games like Smash, Mario Kart and Splatoon can keep selling even higher if the install base is huge. I mean look at Mario Kart numbers on wii 35million it has insane potential. Imagine if Mario Kart was available on xbox and playstation.

You make some very interesting points about the similar sales figures on platforms with very different install bases. Though you have quoted select examples and these things do sometimes change over time.

I think there are SOME Nintendo franchises that are already saturated in popularity (I doubt something like Pokemon would sell much more than it currently does). But what about Fire Emblem, Xenoblade, Metroid? Those would almost certainly increase in popularity several times over if they were multiplatform. Xenoblade Chronicles is perhaps the finest game ever made that didn't sell 1 million copies. 

Ya it seems as though a few games will sell proportionally to whatever the install base is, hard to tell with Splatoon though as it only has one installment. One major reason why I wouldn't want Nintendo to turn 3rd party is that I feel they will stop making some of their more niche or experimental titles and instead focus on the few big ones that push massive numbers since they no longer need to worry about offering a diverse lineup in order to sell hardware, most Nintendo IP are actually outsourced to small 2nd party developers and I'm not sure if Nintendo would keep doing this and instead just develop games internally while publishing titles from a few of their bigger longtime partners like Hal, Intelligent Systems & Game Freak.

I'm not sure about those examples, how many turn-based strategy RPGs sell over 2 million at retail on consoles? Fire Emblem is simply a niche game in a niche genre and likely will always remain that way.

Metroid is one of the games I used as an example in the OP. Metroid II, Super Metroid, Metroid Fusion, Prime 2, Prime 3, Other M all sold between 1.2-1.8 million, Zero Mission & Hunters failed to sell 1 million and the only 2 to sell over 2 million are the original on NES & the first Prime. Over the course of 10 games, the average is about 1.5 million on install bases ranging from 22-154 million. I think the series as a whole is too slow and exploration based to really hit the mainstream. In order to make the game more appealing Nintendo would have to make it more action and online multilayer focused which kinda abandons what makes Metroid what it is.

Xenoblade too is simply niche, there really aren't that many JRPGs that sell big numbers on consoles anymore, Final Fantasy is basically it in terms of big JRPGs that massive worldwide appeal. Dragon Quest is a monster in Japan but niche elsewhere, Tales is a long standing popular JRPG and even that has never crossed 2 million.

All three of these franchises are simply niche and while they have their dedicated fanbases, they lack mainstream appeal for one reason or another.