It is true for all licenses, for example, it is possible to keep old code available under old permissive license but release all new code under a more restricted license.
This not correct. There's a difference between OSS projects with shared copyright (every individual contributor holds the copyright to their contributions) and oss projects where the copyright is required to be transferred to some company.
In the second case, this company then holds the copyright to the entire source tree and can re-license it at will. Mongo and Elastic did this and they are good examples of why you should not transfer copyright because they then have the right to re-license your contributions as they please. That's the reason they insist on copyright transfers. They reserve the right to change the license on future versions of the software. You can still use the old versions under the license that applied at the time.
So, Opensearch is a fork of the last Apache 2.0 licensed version of Elasticsearch. Versions after that are licensed under a non OSS license whereas Opensearch is a proper open source project where copyright belongs to individual contributors, which ironically is still mostly Elastic plus whatever individual opensearch contributors added.
Most projects don't insist on copyright transfers however and given enough external contributors it becomes increasingly hard for them to get permission to change he license.
Regardless of the license, there is absolutely zero chance of something like mysql, linux, or other long existing OSS projects ever being re-licensed because it would require tracing down tens of thousands of copyright holders (or their surviving relatives) to get permission for that most of whom would probably not be willing to do that. This is so impractical that it will never happen. And even if it happened, anyone could continue using and contributing under the old license. All you'd have is a fork that is cut off from those contributions (because licenses like Gpl v2 don't allow mixing with proprietary code). So given a permission you will never get, you'd have a fork that is effectively yours of an original source tree that still belongs to all the original contributors and is licensed under the original license.
OSS done properly builds communities that exist for as long as people continue to be willing to use and contribute to the software. Some OSS projects are now decades old.
I am not a lawyer so I can't keep up the discussion on the necessary level so I will just clarify few things.
Uptrace uses the BSL license to forbid or rather not allow other companies creating a cloud service using Uptrace code, because that is how we are planning to monetize. But you can self-host Uptrace and use it as you want to monitor your (production) application. I think this is fair enough.
I am sure there are many complications with re-licensing, but it happens in practice, for example, Sentry now uses BSL license. And you can't do anything about it except to fork old Sentry. But then you will have to maintain it yourself.
And I am not arguing with anything you've said, but there are not that many financially thriving truly OSS projects. That's why people like me have to complicate their lives with BSL, AGPL, and others.
It is true for all licenses, for example, it is possible to keep old code available under old permissive license but release all new code under a more restricted license.
None is safe! :)