I think the conversion would have been faster, especially for people who don't care, if the main distributions collaborated and stopped providing Python 2.x; 99

I think the conversion would have been faster, especially for people who don't care, if the main distributions collaborated and stopped providing Python 2.x; 99% of software writers would just check out how to write their code in Python 3, or move their code to that version. The other 1% would be people who "knew what they're doing" and custom install whatever version they needed. Imagine if Ubuntu 14.04 (Trusty) didn't bring Python 2. All devs of relevant stuff would have received a shower of upgrade requests. Maybe 2014 was too early to force this, but I'm sure in 2018 it could have been done with Ubuntu 18.04. I know this looks like work for the sake of upgrading, without any real benefit for most projects. But I guess it's not wrong either; you just couldn't expect that Qt 3.3 (circa 2004) was distributed 10 years later in Ubuntu 14.04 (Qt is just a library, not a language, but it practically transforms the way you write C++ so it's a valid comparison IMO); however here we are in 2018, 10 years after the release of Python 3, and Python 2 is still the default interpreter in Ubuntu 18.04.