> However, I'm under no illusion that it'll prevent my unpaid work from being used by billionaires.

There's plenty of evidence that AGPL is reasonably effective at that. SaaS providers are clearly afraid of it.

> not every business model relies on keeping their source code secret.

I was never talking about secrecy and business model.

It's about allowing end users (including myself) to have some reasonable chances to inspect / update / reflash their devices.

Note that the definition of "use" is important here. Many MIT advocates regard shipping parts of MIT licensed code inside a proprietary product as "using" it. And yes, in some ways they are right, but it's definitely not the only meaning of the term.

The GPL has a different usage definition: run and execute. E.g. think of an image manipulation program under the GPL. If you design a game, you run the program to create assets, but none of the created assets contain a trace of the original program any more. The game you ship doesn't either. The GPL doesn't affect you. However, if you tried copying one of the algorithms implemented in the program into your own proprietary competitor, you infringe on the copyright now.