> No, it shouldn't break them if you wrap async methods carefully.
> Here's a metaclass that I wrote recently doing just that

That code does not seem to work for me:
https://gist.github.com/PetterS/f684095a09fd1d8164a4d8b28ce3932d
I get "RuntimeWarning: coroutine 'test_async_with_mock' was never awaited"

@mock.patch needs to work correctly for test methods.

>> I'm really not seeing what a separate class buys you.
> I already mentioned in my previous comments that adding async support to unittest.TestCase would require us to add a metaclass to it, which is potentially a backwards incompatible change.

No, unittest.TestCase can handle this, as demonstrated by two PRs in this bug. This would not change the behavior of existing (working) code.