Python has changed incompatibly once in 26 years, and never plans to do it again, yet you're making it sound like an ongoing pattern. What language are you going to run to that has that kind of track record?

That's not true. Every change breaks some behavior, even if it's not throwing an Exception where one used to be thrown. Things are still being deprecated and removed in new releases.

On top of that, by adding new syntax in 3.7+, libraries may become incompatible with the system-provided installation. That won't happen with 2.7 libraries.

What Scheme implementation do you use that has never broken compatibility and continues to be maintained?

There are tools that use the AST to translate Python 2 to Python 3, such as python-future. (Unfortunately, one of the things the core Python devs botched about the transition is that they put an awful, incomplete, broken one of these named 2to3 in the standard Python distribution.)

Racket's approach is rather different; they just support all the different Scheme/Racket syntaxes and standard libraries PLT has ever supported, with the same runtime. This doesn't include MIT Scheme but there is a syntax for the subset of it used in SICP. I don't think they have anything that converts code between the syntaxes.

I wouldn't mind seeing Python also use that approach -- a Python 3 runtime that runs Python 2 code (and maybe borks your Unicode, which is fine because people who stay on Python 2 clearly don't care that much about Unicode, and that's their prerogative). I'm convinced the only reason no such thing exists is that the first person who makes it would become responsible for everybody's awful old Python 2 code. Maybe in 2020 someone will make it and charge money for it.