The good news is that projects that prevent forking from happening usually don't have huge OSS communities of contributors because of their attitude towards outside contributors. You need an outside community to be able to step up and take over for a fork to happen.
Mostly things like copyright ownership transfer is not a thing with OSS communities because it strongly discourages third parties from contributing. Copyright transfers are only needed with some licenses (GPL style licenses that insist everything else is licensed the same way) and cannot prevent a retroactive fork even if you have them. Other licenses allow distributing mixed licensed code and you can just create a commercial source distribution for those because the license explicitly allows that. Either way, anyone with the pre-license change version of the code can fork. That's why Elastic, which used the Apache license and had copyright transfers, got forked.
The more widely used an OSS project is, the more likely it is that somebody will fork it if it is re-licensed. Because that usually means lots of external contributors and plenty of interest from wealthy companies that depend on it. Meaning there are skills and money needed to fund the fork. Copyright transfers don't stop this from happening. Unless you specifically want to fire most of your user base, this just doesn't make any sense from a business point of view.
A failure to fork basically indicates the project didn't have a strong developer community and big companies simply didn't care about the project.
I consult some clients on Elasticsearch and Opensearch. Most of my recent clients now default to Opensearch. Because it's the OSS option. They are clearly spending money to get support (from me and others) but Elastic isn't getting any. As far as I can see, Opensearch now represents the vast majority of new users and is becoming a significant source of money for hosting, training, and consulting. But Elastic is getting none of that.
My guess is that the industry will learn from the repeated re-licensing and forking and subsequent community split that has been happening. Elastic, Redis, OpenTofu, Centos, etc. The pattern is the same every time: 1) project gets relicensed 2) a few weeks later a consortium of companies pools resources together and forks 3) most users stick with open source and the company cuts themselves off from those users.
Long term, I would not be surprised to see some of those companies offering support for their OSS forks (in addition to their commercial offerings) or even reverting the license change. This would make a lot of sense for e.g. Elastic as there's a lot of duplicated effort between them and Amazon. And Amazon gets a lot for free from outside contributors.
