Do you mean sideloading/custom stores (at least in the US)? That's a good point.
However, the lack of the binaries makes the situation with the OS effectively the same. Once the device is EOL it's e-waste.
This was largely true before already. Once the security updates stop, the hardware is best used as a paperweight. And with practically all phone hardware, only the hardware vendor (and all their suppliers!) are the ones capable of updating the device drivers and various chipset firmwares.
Blame the hardware that relies on binary blobs and vendor-forked kernels that never contribute to mainline.