Presumably yes. I haven't seen any evidence to the contrary.

> and just back up my own bookmarks

Nothing wrong about that. But again.. most people don't find that to be very convenient (I'd actually bet money that that there are is magnitude or a few times more people using Safari/Chrome/etc. to sync their data automatically instead of doing it manually).

I think presuming people want this is like presuming they want 3rd party tracking cookies or that they want their online footprint profiled by the likes of data brokers and palantir and so on. Uninformed consent is not the same as support. Adult humans are mostly smart enough to change their preferences away from convenience when they understand it has bad consequences.