Eight years for Python 3 is a selective example. Much of that time was spent dragging their feet because they were in no hurry.
A much better example is PHP 4 to PHP 5. Now, we can all agree that PHP is a pile of dung but PHP 5 is much *much* better than version 5. And it took just 4 years (first 4 release: 2000, first 5 release: 2004).
The Perl core was already good in 2000. I'd like to have seen what a similar effort without the constrains of backwards compatibility would have been like *without going too wild*.