To be clear, I'm not saying a rails fork is a wonderful idea that will definitely spread... I'm saying it is way more likely to be a wonderful idea than trying to convince rails projects to migrate to a completely different framework.

Sure, you can use a different framework for greenfield projects if you want to. But trying to convince existing rails projects to migrate to a completely different framework seems like a big lift. A rails fork would be much easier of a migration path.

Jellyfin forked Emby, and the open source project gobbled up a lot of the Emby users. Back in the day, MariaDB did the same thing for MySQL. Would it be easier to convince your employer to migrate to a completely different framework, or a fork of an existing framework that you have already built on?

Great for new projects, but what about existing ones? With a fork, there is a path that existing projects could migrate to it.