>Our PMs don't like making things opt-in

That is an implementation detail. What matters is the outcome:

Notion leadership has signed off on this being opt-out.

The calculus here, as you indicated, was that opt-in has little buy-in.

What leadership didn't take into account was the risk of this being publicized, and the blowback from this awareness.

That, or leadership has already calculated that not enough people will care (possibly true).

I suppose it's then up to those that do care to make more noise about this, to tilt the odds?, so this specific calculus (also known as enshittification) doesn't keep occuring (i.e, if the blowback costs are disproportionate to the value provided by default opt-out....)