<tinfoil hat> One could imagine a nefarious state actor offering the author of e.g. uBlock $XX million to get access to a lot of browsers. Not sure about the economics, but more niche extensions could probably be targeted for a lot cheaper.
Not that those are good reasons to sell out your users, but they’re the kinds of circumstances that you can easily imagine happening.
That's why it's so important to have a clean handover way that does not involve handing over credentials: it allows circumstantial sellers to pick a least bad buyer, if it exists. The more visible the existence of a clean path (as in "advertised in the UI vs getting someone at Google on the phone") is the more difficult it becomes to pretend that the shady path is clean. There might even be some "conscience arbitrage", perhaps unintended: buyers who buy through regular handover mechanism, with a believable story of confidence in being able to make clean money (which they may or may not believe themselves), but who then sell dirty. Less money for the original dev, true, but at least there's one handover on record, eroding trust.