That's the problem - they literally do not. Technically better is not a good enough reason to change something. It has to provide some actual greater user value along axes that matter to users, and make the cost worth it. What you are seeing is that python3, well, doesn't. The options in that case are reduce cost or improve along axes that matter enough that cost is worth it.

> Inertia seems to best describe the hesitancy to change.

I'd have gone for 'rigor mortis', but hey, we can't agree on everything ... like, say, a very minor change to string encoding with a 12-year managed rollout.