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.
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.
The goal was to run existing Perl code in the same process, so that you could gradually adopt new features or use existing libraries with new code. If that had worked out, there'd have been less of the Python 2/3 gap.
Then again, the idea that Perl and P6 are different languages is something that's come up after the fact, after it became obvious that P6 wouldn't be ready for general use any time soon.
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.