You're right that back then the expectations were a lot higher, but I don't think anyone even then expected it to replace Perl 5 that fast.

Also, Perl 6 turned out rather quickly to become a new language (although very much in the spirit of earlier Perls), while Python 3 was, as far as I can see (I'm not really a Python person) as an update to fix some important issues in the language, which despite by necessity being backwards incompatible, was intended to still be the same language.