The problem I have with python2 vs python3 wasn't breaking backward compatibility, but that their "solution" for UNICODE strings is such a weird mess (treating string- and byte-streams as something completely separate instead of treating strings as UTF-8 encoded views on byte streams) The only string encoding that matters today is UTF-8, all others are relics from the early 90s and the sooner we get rid of those the better - e.g. Python caused a whole lot of pain for a solution that would have been useful 30 years ago, but not today.
There's another problem that's affecting more than just Python too: grapheme clusters. It turns out that even a sequence of scalars (code points excluding surrogates) isn't always a safe abstraction anymore.
