I will add that at least both PHP and Python core development has been very willing to give any feature that's deemed a mistake the axe without second thoughts. On the opposite extreme, we (the perl5 porters) have mostly taken the opposite stance of supporting real users if in doubt. There are many features of Perl that are widely considered to by ill-suited for larger projects. We'll take very good care to provide a gentle and clear migration path when we break those. I'm sure this helps in migration. At work, we're slowly migrating ~2.5 million lines of code across 14 versions of perl right now. Mostly without a lot of pain.

This being said, this is not a snipe at how PHP or Python do it. It's a choice of the maintainers/developers.