It would be difficult, but it would at least be possible. See projects like Asahi Linux where they did the work to reverse engineer the drivers for ARM-based Macs.
Asahi started to be developed a couple of months after the first of what would turn out to be the very popular M-chip Macbooks was launched. Those Macbooks are still in people's hands in very large numbers.
Compare that to the number of 1st gen iPads or older iPhones and the demand might not be to the same level to justify the endless reverse engineering effort.
The solution is for the companies to have to open up the device once it's officially not supported.
Asahi started to be developed a couple of months after the first of what would turn out to be the very popular M-chip Macbooks was launched. Those Macbooks are still in people's hands in very large numbers.
Compare that to the number of 1st gen iPads or older iPhones and the demand might not be to the same level to justify the endless reverse engineering effort.
The solution is for the companies to have to open up the device once it's officially not supported.