>my tinfoil hat theory is that they make small features depend on new hardware.

The general case is hardly a "tinfoil hat theory". They openly do that, and the major reason is to tie to new hardware adoption.

That said, it doesn't usually work like you call it. It's not adding new features depending on HW optimization to slow older machines down (after all one could just not use those features in an older machine, or toggle them off).

It's rather: you want to get these shiny new features, which is all we advertise for iOS/macOS N+1, and the main new changes? The big ones will only work if you have a newer machine, even though we could trivially enable them on older machines (and some don't even need special hardware, as there are third-party hacks that unlock them and they work fine).

I don't think it's even a broad strategy from PM or higher ups. I actually think it's engineers inside the company who want to play with the coolest hardware and the build features for the newest stuff. Features can be made to work with older hardware but that requires more time and optimization which they never get, so someone takes a call that x and y features only work on newer gen hardware.