So, chromium won't go away. Those 1000+ people are the main resource here. Effectively they work mostly on chromium and not on chrome. What happens to chromium if that stops?
My guess is MS might step up and hire people.
Sadly, it absolutely can go closed-source as it's licensed under BSD-3 clause, which is not a copyleft license.
Technically any form of re-licensing the original implies some kind of fork happens. The old system continues to exist for anyone that has a copy. The license allows anyone to fork of course. So anyone with a copy could just pretend that's the main fork.
Chromium is actually a complicated project. Some of the project coordination is actually taken care of by the Linux Foundation; not by Google. But Google of course controls and hosts the key code repositories. However, they don't own the code base exclusively though because they have been accepting contributions from e.g. Microsoft and the many other companies that base their browser on Chromium. And of course individual developers. Copyright stays with those.
The partial permissive licensing (it's a mixed license code base) makes it possible to incorporate the code in a closed source system. But some of the components are LGPL, which still would require publishing the code of those components. And of course Chrome is an example of such a system and Chromium is the way they publish the source code. And Edge. And Brave. Etc. Chromium is the shared source code base for those. But it doesn't allow Google (or anyone) to take away the copyright from contributors. That would require a copyright transfer agreement. And no such thing exists for Chromium.
So, Chromium is unlikely to stop existing. And if it somehow does, it would immediately trigger a fork with a different name created by all the stakeholders that would require such a thing.
The only question is who ends up employing the Google employees that currently provide most of the code contributions and who owns the Chromium trademark (Google currently). IMHO that's an unhealthy situation that should be resolved in any case. And with multiple wealthy companies using chromium, funding that should be no issue.
> And with multiple wealthy companies using chromium, funding that should be no issue.
You're assuming tech companies aren't being cheap lately when, in fact, they are being cheap. We should assume that the mooching (using Chromium without contributing code or money) would go to 11.