I don't agree, I think you are giving experienced game designers too much credit. If we look at things like races, or sub-classes or whatever other mechanical formulas the game has... there's nothing that has come out that couldn't (or hasn't) been accomplished by regular folks on DMs Guild or Reddit. And in fact, I go through all kinds of stuff on the Unearthed Arcana Reddit and don't see any massive drop-offs between what the normies are creating and what the WotC folks put out. So to put the WotC folks on this pedestal that only they can produce quality game mechanics is to sell a lot of other creators short... plus it greatly restricts what a person can have available at their disposal to use in their games because they're beholding themselves to WotC stuff unnecessarily.
Yes, for our AL players they have to use the "official" WotC products, and there are some other players so paranoid about "game balance" that they don't trust the community who design their own stuff, *OR* their own personal vision to be able to distinguish balanced from unbalanced material that the community produces. So I certainly can understand the desire to ONLY have official WotC products on the table and to then be annoyed when a book that could have extra space for more mechanical material doesn't end up using it.
But then again... we've known for the last four years of the game's existence that WotC is doing their level best NOT to produce too much mechanical chaff and only make smaller stalks of wheat. And thus, we are mistaken in thinking that Xanathar's is wasting available pages by putting in all the proper names and/or re-printing sub-classes and spells from elsewhere (which could have been filled by all-new material)... because in point of fact, they always only intended to include a certain amount of new mechanical material and probably would have just made a smaller book had they not included all that other stuff.
