In this situation, I think Monster Hunter fans should just buy a 3DS instead of a Vita. Monster Hunter 4 was announced for the 3DS before the Vita released, so it's not like fans bought a Vita and then found out that a 3rd party exclusive switched platforms.
A lot of Japanese franchises are migrating from Sony to Nintendo. Nintendo handhelds just sell much better then Sony ones, and Nintendo has a good history with Capcom and Square Enix. If someone were a hard core Monster Hunter fan, they shopuld just switch to Nintendo from Sony.
However, as to the question at hand, for my situation. If there was a great franchise or developer which used to make exclusives for one platform, but later switched platforms, then I would buy the currently supported platform if the game is good enough. However I just might not get the game new.
For example, Jeanne D'Arc and Valkyria Chronicles 2 are two excellent PSP games which I really want to play. So what I did was borrow a friends PSP for a long enough to to let me beat these games. I recently I bought a PSP 3000 used, so I can play other great PSP games and come back to these.
So next gen, I will likely get a Microsoft home console well after I get a PS4 and Wii U, there is no reason why I wouldn't get old launch titles for $20 or less each.