A long time ago, I remember a MySQL talk where they explained dual-license as "if you make money, then we make money", while keeping the product FOSS.

I think it's a reasonable simplification.

And if the company changes the licence, then a A/GPL fork can survive (c.f. Hashicorp, which could have probably avoided their issues with a better licence from the start, but at the time no one expected to be eclipsed by big cloud providers with infinite resources).

While I don't agree with everything Stallman says and does, I really do appreciate that his position on this issue is not just another axiom, but follows purely from other positions he had already established.