Err no, not even Splatoon 2 is a 4M seller in Japan.
And just because it would sell well in Japan, it doesn't need to be exclusive for the Switch.
MH is too big now, they don't need to sell just for the japanese audience, its not like before.
Monster Hunter is a bigger gaming franchise than Splatoon in Japan. It has repeatedly sold 4M on the 3DS and PSP, and Switch is capable of equal or better software sales compared to 3DS.
I agree that having exclusive monster hunter games for Switch does not make sense a priori, but I could see a rationalisation from them that they are putting a smaller team that will not produce a game with production values that are as high as MHW was. As a result, they may want to keep it separate from the MHW line of games targeting the XB/PS/PC. Ideally, they'd put it on everything, though.
As regards your third sentence: a Switch MH game, assuming it is a major MH game like MHG was years before, would have a ton of potential inside of Japan, but also outside of it: just consider the fact that MHGU, a year late port of an expanded edition of MHG, did close to 1M outside of Japan. A new game, presumably, would be localised day one, and have a decent marketing push by Nintendo (and maybe Capcom as well), so it should have a ton of potential outside of Japan, and a MH game for Switch would not be just for Japan (see also
the other post I made on this topic for a bit more expanded thoughts on the topic). Again though, as mentioned in the other post, I stress that I do not think they will do it, nor do I think they would be dumb for not doing it.