Relicensing isn't necessary. If you violate the GPL with respect to a work you automatically lose your license to that work.

It's enough if one or two main contributors assert their copyrights. Their contributions are so tangled with everything else after years of development that it can't meaningfully be separated away.

In addition, there is the potential for software users to sue for GPL compliance. At least that is the theory behind the lawsuit against Vizio:

https://sfconservancy.org/copyleft-compliance/vizio.html

But that's only relevant if AWS (in this example) violates the GPL license, and it doesn't really seem like they have?

They can switch from LGPLv2.1 to GPLv2 or GPLv3 for future development because the license has an explicit provision for that.

I don’t know about ffmpeg, but plenty of OSS projects have outlined rules for who/when a project-wide/administrative decision can be made. It’s usually outlined in a CONTRIB or similar file.

Doubtful that's enough for a copyright grant. You'd need a signed CLA.

No one said make it proprietary; there are other OSS licenses that would make ffmpeg non-viable for commercial usage.

You need a copyright grant to change the license in any way.

(Except for the part in the LGPL that lets you relicense it to later versions.)