So, let's force that "value" on every user rather than let them build the experience they actually like?

Sure? Companies do get to dictate things to their customers: they can build things, or not build them. Customers don't get that control: their recourse is to the market.

They absolutely should get that control though. The only thing preventing them from having it, often, are artificial limitations. Don't place too much faith in "the market" to fix problems.

I don't like YouTube's UI therefore I should… use Vimeo?

The biggest plus is that — like Chrome, if they decide they made a mistake, they can pivot a change that many of their users would disagree with, and force it on the whole ecosystem immediately. Chrome has done this a lot, and almost all of it has been security-related stuff that's been jammed down our throats. And sure enough - the medicine has been quite distasteful, but it was good for the patient.

A lot of this stuff is UI-related; it's things that aren't a reskin, but they're a major shakeup of how the entire UI works, and would break all other clients using a framework.

The problem with "truly" federated protocols is that every design fuckup is preserved in amber for eternity.