For most devices, supporting a device indefinitely is just a matter of letting the user flash their own firmware or replace the existing one using an SD card.

Using your devices for a long time is not possible not due to the difficulty of the community maintaining the software, but because the original company put user-hostile signature checks on the firmware.

It’s not realistic because tech companies have conditioned us into that expectation. My dad has a 40 year old high end sound system which works flawlessly. Why can’t Google maintain some servers and push some fixes for 20 years?