People distribute code via pypi.  if we reject uploads of packages with these problems and link to fixers (modernize can be taught what to do), we prevent them from spreading further.  A few years after doing that, we can revisit how much pain and for whom making this a SyntaxWarning or even SyntaxError would actually be.

it isn't useful to tell users of packages to spend time figuring out who to complain to that some packages code that they somehow depend on (often transitively) is not modern enough.