Fedora ponders the Python 2 end game

I expect the point dropped is that every compiler toolkit will compile both for N years. The C compilers would work with both K&R C, ANSI C and C99 while emitting warnings. They would then drop K&R C or make it require an explicit flag. It then would only emit warning for mixing ANSI and C99. Then ANSI needed an explicite flag and then C99 only. All in all it pretty much took 15 years from C99 for various compilers to transition.

The same with Fortran. You could mix and match IV and 77 code. Then you needed a flag, then you needed a flag for F90 etc etc. And the same for C++ code. The transitions for each were slow and Azathoth knows how many compiler writers went mad having to work that kind of code. Which I expect that Guido was trying to avoid.