I think the crux of the Monster Hunter 5 platform argument is the following. In order to not debilitate Monster Hunter sales in Japan (and thus lose the vast majority of the sales on Capcom's best performing franchise), Capcom would be crazy not to have a portable SKU. Basically even if Western sales were to increase (MH4U did decently on 3DS in the West), it's VERY VERY unlikely that it would even come close to the #s MH has gotten on 3DS and before on PSP. It might have something like a 50-60% drop, which would be horrible for the franchise.
Thus, if Capcom is sane, they'd launch the game on at least one portable.
The options then are:
1. 3DS - It's worked well over the year, but this isn't a long-term solution.
2. Switch - being the 3DS's successor, this serves a better long-term solution as essentially the only portable going forward, but it still has to build the userbase (which wasn't a problem for the 3DS though and doesn't look like it'll be a huge hurdle for the Switch thus far).
3. Vita - Capcom ignored the Vita back it was much more viable than now, and it's even more on its way out. This is super unlikely (yet still would probably be better than doing a PS4 exclusive imo simply due to portability lol).
or some combination of the three.
Multiplats are totally up to Capcom and shouldn't hurt TOO much. 3DS/Switch would be similar to MH3U (so there is some precedent). Switch/PS4(/PC/XB1) could probably work (Switch acts as base SKU and then put prettied up versions on other platforms). 3DS/PS4 would be 2 totally different games (but then faces the future handheld problem, so why not just put it on Switch now), and 3DS/Switch/PS4 I guess could work, but that's a lot of work lol.
Essentially if the game doesn't launch on Switch or 3DS, it's going to have problems unless someone figures out how to make the PS4 portable =P.