I think Linus is too stubborn with his willingness to remove existing ABIs. Removing an ABI should be about as hard as adding one, because both actions change the ABI. If a new ABI is introduced then userspace can't be sure it will be there and has to check for it anyway. So the sooner you remove a bad ABI, the better, because less userspace relies on it being there. It doesn't matter if an ABI doesn't work because the kernel is too old, or because the kernel is too new. Waiting 5 to 10 years is the worst possible approach, except for ABIs that are twice that old.