Amazon doesn’t care what you do, piracy isn’t a threat to Amazon, they just want to deliver the drm that the publishers demand in the easiest way possible. And they’ve made a fairly reasonable business decision that a customer who hasn’t bought a new kindle in ~15 years isn’t an important source of revenue worth supporting old devices for.

And yeah, it sucks. But it’s the ecosystem you bought into when you bought a kindle. This is what drm does. If you care about open systems, support them.