The new ASDF will break systems that subclass OPERATION. Those libraries' maintainers will have to look at their code, based on the error message, and see what needs to be done. For the vast majority of them, five minutes work will suffice. For those that take more... well, as you point out, the bug fix had to be made, and I stand ready to help those who need assistance adjusting.
It's good to signal an error or a warning for this case.
It would have been better if asdf:operation would have kept its old semantics, maybe with a deprecation warning when it's used, and that the new semantics would have been provided by, say, asdf3:operation. People who see the warning and care about keeping things up to date could react to the warning and move from asdf:operation to asdf3:operation, which is not much more work than what is necessary now. People who care less (maybe because other things are more important - asdf is not the center of the universe after all) would not need worry, because everything would work as before, even in a setting where both asdf2 and asdf3 semantics would be needed side by side.
Pascal