But apparently Python needs some "success" and this makes a good bullet point. Correctness does not really matter in the Python world.

The unicode/text changes alone were a pretty good reason. Division producing floats are also a nice change IMO. I don’t want to discount the challenges with the transition but saying there was no good reason isn’t right to me.

That is too long for Python to have 2 modes. Half a decade is more than enough time for 2 modes to become the status quo. For one thing, think of all the outdated Stackoverflow threads that will be hanging around after that 5 years. I am not optimistic that 5 years won't turn into 10 years of uncertainty and breakage.

But 5 years might be too short for everyone to dredge up all that C code, update it, test it, and call it mature.

I guess we'll see.

Developers are lazy. Most people will just do nothing until, 5 years from now, some blog post will go "oh, next month we switch noGil as default, good luck!". At that point everyone will scramble, rush out buggy releases, and spend 5 years finding all the problems.