This topic has been beaten to death in various venues and there is no satisfactory answer for everyone. We have reason to believe Apple will never drop this on current hardware (for legal among other reasons) and have no interest in dropping this for future hardware. Third party OS support for Macs is, at this time, strict internal corporate policy at Apple. They have spent millions on staffing to make this possible by developing the entire Boot Policy subsystem, which does not exist on iOS.
If you have concerns nonetheless, then it’s your choice not to purchase this hardware and not to use Asahi.
In the hypothetical and, as far as I’m concerned, extremely unlikely event they phase out third party OS support for newer Macs at some point, then we just stop new SoC development and concentrate on the existing machines.
If they try to remove support for existing machines forcefully (e.g. by removing it in a macOS update and ceasing to sign older updates and blocking them out of Reduced Security mode too - which is a completely ridiculous sequence of events and walks back on the entire design of Boot Policy) then they will get sued for it, just like Sony did (and lost).
Any computer manufacturer is (from a technical point of view) free to remove third party OS support at any time. All those UEFI x86 machines that ship with the Microsoft signing keys could, at any time, ship a firmware update (delivered via Windows Update) to lock the machine into Secure Boot and only the first party Microsoft keys. Apple is no different here. So if you are worried about this potential situation, you’ll have to limit yourself to open-firmware machines, of which there are very few; neither Apple nor the vast majority of the x86 ecosystem are free from this hypothetical doomsday scenario.