Re: anything but python
> Yes, but by & large they have backward compatibility.
Yes, but it is only "by and large". When moving from one version of C++ or Java to the next there will always be some issues which need resolving, except in trivial code.
Python3 is a new version of the language designed to be a significant improvement. Python2 is still developed and supported and has 'futures' and other tools to ease the transition to the new language. This has been done by numerous languages: extreme examples are: Pascal to Modula2; VisualBasic - numerous times;
Python3 vs. Python2 should be compared to Kotlin vs. Java. Kotlin is designed to make Java into a modern language and drop 22 years of baggage that it still carries. C++ has 36 years of baggage.
> "You can write K&R C in any language" (including C++ last time I checked)
Actually you can't. K&R C (edition 1) was replaced by ANSI C and few modern C/C++ compilers support the original K&R (though gcc may still do so). And that is hardly "any language".
And I don't know that anyone said that; what they did say was "You can write FORTRAN programs in any language", which is quite a different thing.