I don't see where GP made a claim that it would. Why would one expect code written for a newer version, using features that don't exist in an older version, to run under the older version?
Folks using u”” were already being careful with Unicode . Supposedly Unicode was important enough for 3 to default to it.
The cost benefit of then destroying code using u”” made no sense to me. It was unnecessary. This was followed by the oh just upgrade thing. I’d love to see the “quick” upgrade guidos employer did. My guess is a total lie that it was easy and it probably took years and VC type money.
It’s not about using new features, it’s about not unnecessarily breaking old or allowing for cross compatibility.
Except they broke the old way for existing features, required you to use the new way and in most cases you could manually write code so it would still work with both. Meanwhile the only tooling provided by the python 3 crowd screwed over anyone with an existing customer base stuck on python 2, which at least early on should have been a foreseeable problem. Not to forget that anyone stuck on python 2 was literally Satan and projects where outright "shamed" to drop python 2 support.
Many folks would have been ok updating to 3 compatible approaches if it didn’t blow up their 2 story. That’s what ultimately happened in lots of cases when it became more doable. The rip and replace everything at once was a weird goal