You see, I see this as a mis-step, and I think it's important to discuss such missteps. But I don't know enough about the internals of Vivaldi the company to know whether the company has bad attitudes and this is yet another manifestation of a draconion culture, or whether someone just tried doing The Right Thing (modern edition) and ended up legislating unintended consequences.

In other words, I'd like to assume the best intentions, and they might yet choose to rectify it.

Looks like a standard vendor lock-in tactic to me. It makes it harder to switch browsers.

But they all do this. Passwords and bookmarks that can't be exported or transferred easily are the usual suspects

Vivaldi seemed big, bloated and slow the last time I used it.

There's no lock-in here on the desktop version of the browser, even if they were big enough for that strategy to work.

Passwords and bookmarks are freely exportable, and even if they aren't in the future, they can be transferred to another computer or extracted from the profile data without issues.

Using Vivaldi hardly ties you to the mothership. At most, you'll lose sync access via their service, not brick your browser.

Correct me if I misunderstand but they're not threatening to brick your browser, they are threatening to brick your email.

Yes, but even if they brick your email, your local browser profile or its data export so you can move elsewhere won't be affected.

Losing access to email and contacts can have a big effect on some people.