...placing iconic D&D monsters into the space for players to fight, but then not having them actually be those creatures; it adds nothing of extra value to those who don't know better, and only makes those who do know better annoyed [...] So why; why in the world would you support decisions from them that make no difference to some people's enjoyment and take away from others, and add no positive value experience over a difference choice, to anyone? Because that is what we have here..
I'll emphasize this point. Regardless of how much leeway you think Larian has as a DM (from "obligated to run 5e RAW" to "can and should change anything including use of the d20") Larian's goal should be to make a better, more enjoyable game. Any decision they make should be with this in mind. "Better" of course depends on a lot of things - more enjoyable, less work, less offensive, thought-provoking, etc.
If X% of players dislike a change, and the remaining Y% don't care about it, then said implementation strictly harms the final product and the decision to implement that change is bad. Now obviously there's some non-zero number of tabletop D&D players who
like Larian's monster changes. But from what I've seen on this forum and elsewhere, the PnP players who dislike the homebrewed monsters significantly outnumber PnP players who actively like the changes (again, people who are indifferent don't matter).
As to why: e.g., I know that Mind Flayers and Intellect Devourers are super terrifying enemies in PnP. So in BG3, I start out super cautious and wary of them, making encounters with them more tense and interesting. And then I realize they're ~pushovers lacking their signature abilities, which is disappointing and remains disappointing any future times I encounter them.
