Can you imagine if the PSF had been working on Python 4 in parallel, with more backwards compat shifts or even a radically modified syntax?

I think another saving grace was, when considering Python 3, one's choice was between "easy-ish migration to best in class" and "difficult rewrite into second-best". Meanwhile with Perl 5/6 it was "two moderately hard migrations into metastasized shell-script has-been language" and "difficult rewrite into best-in-class with lots of upside".