> I think it's clear Apple intentionally* degrades software quality on older iphone/mac models in order to drive purchase of newer hardware.

I don't think that's clear at all.

> * By pushing software updates optimized to new hardware with obvious regressions on older models.

New software can come with increased hardware requirements, yet customers want to benefit from new software. It's a compromise. You could simply not update if you're worried about that.

> I've been considering switching away from Apple hardware, not because of hardware quality, but because of software.

I'm sure software updates long after your device's release won't be a concern with the competition. You probably simply won't receive any.