[jillesvangurp]

The license matters less than copyright ownership. Prior to the license change, MongoDB insisted on copyright transfers. Every line of code was owned by them. That's why they were able to re-license it. Elastic, which used to use the Apache 2.0 license, did the same thing: they insisted on copyright transfers as well.

Other people that didn't own the copyright to any mongodb source code of course had the right to take the source code and fork it under the AGPL. But there would have been no choice about the license under which to distribute that fork because of how strict AGPL is. By insisting on copyright transfers, Mongodb was able to dodge that and re-license the entire code base without having to require permission from anyone because they owned all of it.

For the same reason, there never was much of a community of contributors outside of Mongo. Most large companies would have steered clear of that legal mess and declined to contribute or fork. The flip side of course is that this strong ownership marginalized mongodb as a community even before the license change. It simply didn't matter much to most large companies as they would have steered clear of it anyway.

With Redis, this is not the case. Redis the company was an active contributor to the code base but most of the contributions actually came from the outside and they never owned the copyright to those contributions. The BSD license allows anyone (including Redis Inc.) to redistribute the code under whatever license. Which is why Redis can do this. But for the same reason everybody else can continue as is using Valkey without having to worry much about Redis the company having retired from what otherwise is a thriving OSS community.