I don't think that's true at all.  PS4 version of Resurrection looks to be selling on par, maybe a little below Rage Burst.  Which indicates audience hasn't expanded at all for this type of game in ~ 8 months.

You can't really look at proportions.  Vita version was bound to drop - it's a remake/port of a game which is a) readily available on PSN for Vita and b) was part of a UMD passport program where you could get the game for free on PSN if you owned the UMD at the time GE2 released.  The pool of people who are willing to re-buy something like that on Vita is bound to be much smaller than on PS4 where the game isn't already available.

Keep in mind that PS4 has all the cross-play SKU's and those off shoot games and I think the anime as well bundled in across various versions. 

Even if you are making the "fanbase is migrating" argument, it's going to take more than a few thousand sales moving from Vita -> PS4 to sustain a franchise which regularly sells 400-600k.

I don't think hunting games are properly going to take off on home consoles at all.

I suppose we'll know for sure after the next mainline entry releases, if the fanbase actually is migrating.

Japan is not showing migration but it is definitely what Sony is aiming for. There is no reason to develop GE3 just for the japanese Vita audience so therefore GE must continue existing in PS4 and the only way is going global. I don't think Sony is trying to just milk with GE:R but preparing GE:R+GE2:RB global release to pave way for full scale PS4 GE3.