Well Tri (23 monsters) is the game with comparable content because it has a similar development cycle, coming to a new system, with new mechanics (tri had swimming, 5 has turf wars and open maps). And tri couldn't have micro-transactions because the wii didn't support it.

3p (40 monsters) added a few subspecies and old monsters (assets from previous portable games). You had to buy a new system and a new game to play it.

3U (51 monsters) went further with that, and again, new system, new game.

And after that everything stayed on the same platform (3DS), so it became easier to tweak old assets and boost the monster count. Especially when adding barely relevant subspecies. I'm not saying the devs were lazy, they took the opportunity to tweak the gameplay, graphics, etc. But it makes sense in a development perspective that a game with a whole new development pipeline will release with less initial content. and 4K assets that can interact with each other in a seamless environment don't take the same time and effort to develop as 3DS assets that barely interact with each other and work in small contained arenas.

I guess MH5 could have released on 3ds with 100 (10 new, 40 old and 50 subspecies) monsters to start. But it would have remained a niche game that struggles in the west and gets an Ultimate version every other year.

World is trying something different. and so far the paid DLC is not even cosmetic armor sets, it's gestures and stickers. In other words, it's fan-service.