I am not privy to internal discussions, but it seems to me if there is really lots of ongoing maintenance needed at every release, and not simply exaggerating the need for occasional patches and bug fixes, then it is worth some consideration of re-architecting the way the compatibility mode works.

eg. Maybe draw the boundary of where you thunk at a different place. Maybe do more translation in user mode as this wine project did. Or focus on binary compatibility with old, frozen-in-time frameworks instead of rebuilding them with every OS build. Obviously the specifics will vary with actual details of breakage. But from outside, it seems like these type of questions weren't pursued to their maximum extent, resulting in pain for the end user.