Using XIVDB, I've counted 2,800 "development abilities" (monster skills). Subtracting the few that are untranslated from Japanese for some reason (maybe they're never displayed?) and the duplicates and the mount abilities and so on, it's more like ~2,600. That's a lot of new terms to add. I suspect that's what the dev team is hesitant to include.

That said, only a small number actually matter. But, how do they determine what "matters?" I feel like there are at least a couple hundred that really matter for the fights they are a part of, and while you can argue that those fights aren't important, they likely are important to someone. For instance, the final boss of Toto-rak - do you include the move where she spawns an explosive pod on top of someone (I can't recall the name - silkscreen or sticky web)? What about the second bosses of Haukke Normal, who have two interruptable skills that are nice to be able to talk about if you're explaining them (Ice Spikes and ... soul drain, I think?), and the final boss has that dodgeable targeted attack Void Thunder III, and it's not obvious you can dodge it by running behind her. And what about non-bosses? "Self-Destruct" is important to know to avoid in early dungeons where it's an instakill. "Final Sting" is relevant to anyone explaining Qarn.

So, in other words, it's really a value judgment regarding what to include or leave out, even if you're just putting in abilities that are useful to be able to mention. If you're only adding stuff from current content, that's one idea, but that's 1) still a lot of slots, and 2) another value judgment about how current is "current," and when to remove them. Also, when 3.0 comes out, there will be hundreds or more abilities from "current" content. Do all of those get added too?