Not only is this untrue in the case of Python, I think it's untrue in the case of basically every other successful language: - ANSI C was incompatible with pre

Not only is this untrue in the case of Python, I think it's untrue in the case of basically every other successful language: - ANSI C was incompatible with pre-ANSI C. Current C has various minor incompatibilities like the removal of gets(). - bash defaults to non-backwards-compatible behavior and lets you set various shell options like "compat31" if you want it. - Newer version of the JDK generally require code fixes. - PHP has made various backwards-incompatible changes, like the removal of magic quotes in 5.4. There's also many more cases of languages getting forked in backwards-incompatible ways and going on to be successful. C++ is doing fine despite not being fully backwards-compatible with C; VB.NET broke compatibility with VB; zsh isn't POSIX-compatible by default; there were a million variants of BASIC back in the day, and still are a million variants of SQL; etc.