20 years after. It took 2 decades. My impression from the debate, and taking in consideration the political context, is that he saw that as the tipping point of the BDFL transition and a good test run as much as a language feature.
I'm not following what you're saying it's a test run of. I agree that he probably wanted to cease being BFDL because it's a big job, but with the caveat that I'm terrible at following Internet drama, he did seem genuinely surprised at the outrage.
Regarding that it took 25 years, the normal Python syntax has been enormously successful. People don't tend to look for problems in things that work.
So one might argue that a principle of python is that the language is dictated by how people read and write rather than the other way around... it's pretty hard to say what principles are "core" when they all conflict and you have to weigh various tradeoffs.
[1]: https://www.python.org/dev/peps/pep-0572/#the-importance-of-...