[Pet_Ant]

Because you end up accumulating so much cruft and complexity that the language starts to fold under its own weight.

Often you hear the advice that when using C++ your team should restrict yourself to a handful of features. ...and then it turns out that the next library you need requires one of the features you had blacklisted and now it's part of your codebase.

However, if you don't grow and evolve your language you will be overtaken by languages that do. Your community becomes moribund, and more and more code gets written that pays for the earlier mistakes. So instead of splitting your community, it just starts to atrophy.

Python 2 to 3 was a disaster, so it needs to be studied. I think the lesson was that they waited too long for breaking changes. Perhaps you should never go too many releases without breaking something so the sedentary expectation never forms and people are regularly upgrading. It's what web browsers do. Originally people were saying "it takes us 6 months to validate a web browser for our internal webapps, we can't do that!" ...but they managed and now you don't even know when you upgrade.