FYI I have 3 projects that use pth files to activate various features (an env var is usually the trigger):

https://pypi.org/project/pytest-cov - enables coverage measurement in any subprocess
https://pypi.org/project/manhole - installs a debug service
https://pypi.org/project/hunter - installs a tracer

I wouldn't like them being rendered almost or completely useless by such a hasty change. 

Running stuff during startup can be problematic and tricky, for example I have painfully found out that on python 2.7 you can completely hose up your codecs registry if you try to decode things during startup (before the registry is fully built) but I think it's a fair price for such a powerful feature.