sure, "contributing is cheaper than maintaining a fork" is true most of the time - but the moment new Microsoft comes in with "embrace, extend, extinguish" (or just copy and change), you're doomed
and heck, we had that exact thing happen last autumn, iirc - making big news on this website
You're making a big stretch here. Sure, you can be left in the dust behind their proprietary fork, that's true: https://hypercritical.co/2013/04/12/code-hard-or-go-home
But your habitual workflow isn't "doomed". You can always fork and keep using the same open version of the project that you've always used. If the project is popular enough, there's usually a community that keeps maintaining that fork.
That's the deal that you get. Free software was never about "free upgrades forever". It's about the freedom to fork.
I never noticed the word "fork" in the GPL. You may want to reread it, as I think you missed the point.
If it ain't GPL and it IS popular, I think those kinds of things are more likely to happen than not.
Even GPL can't force a company to maintain and keep developing an open version when the company doesn't want to. Even if Redis was GPL (no CLA), they could still abandon it and write a compatible clone from scratch. AI makes it even easier to do
microsoft spent a lot of time asking questions to the author - and then rolled out Azure copy of the lib
author went public "how dare they use me like that" - but he did make it BSD himself, it's his problem