Dropbox did it for Zulip.

It entirely depends on how much the seller cared to ensure continuity. It’s not like VoidZero didn’t have plenty of leverage; they weren’t a dying open source project.

Dropbox spun out Zulip because it was a failed project they were going to shut down.

This is completely different?

Dropbox never had any intention of running Zulip. But the team wanted Zulip to continue existing, so they worked that into the agreement with Dropbox.

That is my understanding.

What would they benefit from taking it closed source? And how would that even work for a JavaScript toolchain?