[conradfr]

I would think that the PHP4 -> PHP5 (relative) slow transition was more about hosting support than developer's adoption. Because of of the extreme (and often infuriating) stance on backward compatibility migrations were not that difficult : http://php.net/manual/en/migration5.incompatible.php

It was mostly new things, not different things. I don't even remember the new object model being that hard to pick from the PHP4 one. I think lots of developers got more stuck on the 5.2 -> 5.3 transition.

Isn't Python 2/3 more about libraries and compatibility than hosting ? My personal anecdote is that it deferred me way too long to learn the language seriously. I didn't want to waste time learning an old version but too much things were on it, especially Django and Flask (being mainly a webdev).

PHP7 vs Hack could mirror that though.