They probably have no choice. I dont think they can afford to do a MHWG in a year or whatever. This game needs to saturate.
While I do think GaaS is the logical direction for World (though you can read my confusion on the seeming split of MHW and MHX pipelines and products in the MC threads and how that conflicts with GaaS), there are some issues with it in this context that I don't think have been deeply considered.
If the monsters themselves are part of GaaS as a DLC component, much as maps were in shooters a few years back, then you're going to face a potential fractal splitting of your audience. In the G-version releases, the split for the community was binary, you're either in the extended release or youre' not. Here it becomes potentially fractal if it follows a release type to that of SFV which would probably not be an ideal hole to go down. Each monster beyond the base becomes a fragmentation of your online community if it is sold piecemeal.
However, there's not much in this series to offer for GaaS in terms of the usual monetization methods unless they are introduced *for this entry*. Loot crate armor skins, boosters, flare, or whatever, to subsidize monster additions much as how Halo now gives maps out universally for free but monetizes other aspects like Warzones.
They'll have to be smart in how they handle G or even basic monster additions. GotY may be the best option but Capcom as a whole has moved to GaaS multiplayer centric titles, so I don't know how they will handle this.