With them changing to BSL, all updates to the MPL version stopped immediately when the change was announced. Your runway is now exactly up to the point where a critical vulnerability is discovered and puts your project and your clients at risk. Or you tread into uncertain legal territory and hope that they don't try to smite you while you figure out what to use next.
That can also happen without the company going out of business, the community's efforts simply need to be based on the last version before the license change. See for instance how OpenZFS development continues even though Oracle is still around.
With Hashicorp, the project is still "open source" but under a license that makes it potentially dangerous to any competitors. That makes it much harder for a fork to operate without exposing itself to legal risk. For example if any of the code implemented on the fork after the change was too similar to code by Hashicorp that could open the fork devs up to a legal battle.
With ZFS that was never a concern since the new changes weren't publicly available on the closed source version and importantly all the brains behind the project promptly quit to work on the open source version instead.
