Thanks @gpshead for proving my point about clear messaging. 
The plan is absolutely not to complain about packages that don’t have public wheels on PyPI. You’ll still be free to publish packages with only an sdist. What we want to change is that installing those packages will work by building a wheel using compilers etc. on the user’s system, and then install from that. With setup.py packaging, that means running setup.py bdist_wheel in the install process. This is already what pip tries to do, but there’s a fallback to running setup.py install if it fails. The discussion is about deprecating and removing that fallback.
We believe that very few packages would be affected by this. If your package is a counterexample (setup.py install works but setup.py bdist_wheel doesn’t), please share details!