There are some parts of the software that are specific to the AP2 hardware and some parts that are common across all hardware configurations. So if Tesla actually fixes the multitude of bugs in the media player someday it's not unreasonable to expect those to be rolled out to the entire fleet. Chill mode and speed limits, two recent features, work for all AP cars (as far as I know). I would not, on the other hand, expect EAP/FSD features to roll out to AP1 cars.
Certainly I don't expect much further development on the Autopilot functionality itself for my AP1 car, at least for those parts that are dependent on the hardware platform. But it wouldn't be acceptable to break working functionality on currently-deployed cars. One strategy might be to keep older hardware platforms on older software release trains, although that incurs other support headaches, and Tesla's deployment strategy has in general been to try to keep all cars running the software software versions.
So far this thread hasn't touched on how Tesla handles the pre-Autopilot cars. Note that they're still getting software updates several years after manufacture. Obviously they don't reap the benefits of Autopilot improvements, but the newer software (aside from bugs) generally doesn't introduce any obvious regressions in behavior compared to the old. That is a realistic expectation.
Bruce.
PS. As I read back over this I'm not sure what your definition of "support" is. To me it means "make sure it works". Not sure if you mean that or "port all the new features and functionality to it".