One serious downside (for end users) of having a stable driver ABI would be that manufacturers would release a binary blob driver for their hardware which just about works, but no bug could ever be fixed. Manufacturers wouldn't have any incentive to fix bugs and kernel developers couldn't do it either.
The situation of GPL'd drivers in a kernel with an unstable ABI wasn't really planned, but for users it's probably the best outcome.
The situation of GPL'd drivers in a kernel with an unstable ABI wasn't really planned, but for users it's probably the best outcome.