The new policy should apply to answers that predate it
If the community decides to remove invalid answers (and it certainly looks like it right now), that policy should apply to all answers. Reasons for this include:
We are not changing the rules.
For a policy that invalidates previously valid answers, retroactive application wouldn't be entirely fair, but that's not the case here. The answers were always invalid. The only thing that is debated now is how to deal with them.
Old or new, they are low quality.
While there was no clear consensus that invalid answers should be removed, there always was consensus that they were low quality. Low quality contributions are and always have been subject to removal. In fact,
The answer is extremely low quality: There is little to no scope for improvement
is one of the suggested reasons for deletion for trusted userstrusted users.
There is little to no rep loss.
As explained in Reputation and Historical Archives, any answer with a score of +3 or higher that has been visible for 60 days or more will not cause a reputation loss if it is deleted.