This issue has been discussed during the Language Summit. A quick poll showed that the majority is in favor of changing the default in Python 3.9.

Lukasz proposed a PEP update to propose to switch the default in Python 3.9:
https://github.com/python/peps/pull/1371/

For me, the unclear part is which projects would be impacted if the default changes?

Someone mentioned attrs, but it seems like attrs is fine:
https://github.com/python-attrs/attrs/issues/288#issuecomment-587265961

In term of workflow, I would _prefer_ to get such incompatible in the very beginning of a devcycle, rather than just before the feature freeze. But I don't think that it's a blocker issue. Technically, changes are allowed until 3.9.0 beta1. Moreover, Lukasz is the 3.9 release manager, the author of the PEP 563 and he is in favor of changing the default in 3.9 :-)