I think I do understand as to what you might meant by "become useless" after you've upgraded to MM. No offense and sorry for a wall of text. I just want to share my thoughts.
The most reported issue for the MM version of AF2 was with the camera not optimized for 3rd party apps.
And yes, AF2 was marketed to be a "mobigraphy" device which in my opinion is still is since the default camera app works best. And yes, that statement does not help at all so I'll just share some insights when it comes to development.
Please do not take this from me as I am not making this in part for the official Flash team's whereabouts.
Android itself comes from the AOSP (which is open source, so anyone can have whatever Android they want it to be) but most manufacturer have ties with Google that is why most have additional features in connection with Google-bundled services.
BUT that is not enough to make the phone do its thing since it requires HAL as most of you know that one phone is different from another depending on the brand/manufacturer --- this is similar to a laptop or PC, so those of you who have built a PC before would know that after installing the OS, you'll need to install drivers for specific devices/components for a particular OS (if it is Windows) and particular Architecture (x86/x64).
So in Android, where does those "drivers" come from? In techie terms, it is the BSP or Board Support Package on which chipmakers provide to manufacturers. This BSP is a very confidential package of proprietary components which will be the heart of a phone's functions ASIDE from the software services offered by Android (or Google, like the Play Store).
Just like in Windows, you won't be able to use a usb camera if the driver wasn't installed properly, and if the maker of that camera didn't update the drivers for Windows 10, then you'll be better off staying with Windows 7 just for the sake of that camera. This idea applies similarly to Android, if what the manufacturers received is solely for LL, then they won't be able to provide an upgrade to MM right after it was announced. This is also the same idea as to why adoption for a new Android OS takes months (or in some cases, years).
What happens between the chipmaker and manufacturer when it comes to those proprietary things are internal. The manufacturer on the other hand has to deal with certain things as well since it all depends on certain aspects of what the market (country) allows. This is similar as to some of you might have read that EU versions of AF2 have no plans of getting MM.
I just hope that the development goes smoothly so that we can get the update before the Flash team gets overwhelmed with negative feedbacks and decided to drop it since I heard that they already are working with MediaTek in regards to this camera concern for 3rd party apps.
Also, once the update comes, I hope that it also includes the October Security Patch.