I definitely wouldn't rule it out.
Like we shouldn't rule out the Vita getting a mainline Monster Hunter?
Look I am all for multi-platform releases, but I don't see any reasons it makes sense for a series like Monster Hunter.
This is a Japanese sales thread so I am talking from the perspective of the Japanese market.
Lets take the biggest points that make Monster Hunter successful.
-Local Co-Op Play
The main success of this series is derived from its pick up and play with whoever wherever you are. That success has a couple of requirements to keep it going.
-Portability (Play Wherever)
-Large User-base
The portability is essential for a game that relies on local co-op, which is one of the reasons we have not seen a mainline Pokemon game on consoles as of yet.
On the other hand you also need a large and active user-base for a game that relies on local co-op to be effective.
Multi-platform impedes what makes Monster Hunter successful. It would basically lead to one of two outcomes a split in user-base which would undermine the principle the series became successful on or a very small portion of users would play on one platform while the vast majority stuck to the other platform.
This brings us to needing a large user-base. The Vita/PS3/PS4 are never going to be able to support the user-base that the 3DS and most likely Switch will.
So from a sales perspective why would I want to release a multi-platform game that will either
A) Split the user-base and undermine the success of the series.
B) Have a disproportionately small user-base that I need to continually support ect...