Even if Python 3 emerges as this great thing, it could still stunt the Python ecosystem. Python (the thing) is distinct from Python programming, there are plenty of amazing languages that no one uses.
Rebol is doing better than Perl 6.
Python 3 occasionally prevents work getting done in Python 2. That's it.
Perl 6 isn't stifling Perl 5, at least not anymore. Since Perl 5.10, the releases have been fast and contain many new features.
"Stifling" could refer to technical aspects of the language and its ecosystem or it could refer to adoption and marketshare.