The Vita would be in a much healthier state today had it got a mainline Monster Hunter. But instead Capcom put all their eggs in the Nintendo basket.
Maybe. Or maybe not. But why point your finger to Capcom, which made the most reasonable choice back then? By bringing MH on 3DS, Capcom also knew it would have full support from Nintendo in distributing and promoting the IP in the West, something that is actually paying off; I don't remember Sony ever recognizing the importance of the IP by advertising it in the West.
A market where both Nintendo and Sony platforms enjoyed success would've been much better for the industry.
That's highly debatable. Game companies have been migrating towards mobile since before PSV was released; Nintendo has not increase the price of its platforms, and you still get MH clones on PSV. Also, Nintendo was basically alone with both GB and GBA, and the industry was just fine.
Yes, you can argue that it's not Capcom's responsibility to save Sony, but in the end they could have done it, and then have two successful platforms to make games for, instead of one.
Splitting the userbase is something Capcom wanted to avoid. Also, Capcom was also interested in establishing the IP in the West.
Of course this all assumes that Nintendo did not pay Capcom for exclusivity, and that Capcom made this decision from their own accord, which may or may not be true.
I like this line of reasoning that when a third party is going to Nintendo, it's because of money-hatting. Well, true or not, Capcom's decision payed off and 3DS is becoming one of the best-selling consoles ever in Japan.