I think with Python and Ruby the assumption is that the typical code base is either in a state of permanent change or dead and thus the advantages of improved c

I think with Python and Ruby the assumption is that the typical code base is either in a state of permanent change or dead and thus the advantages of improved code / improved API outweigh the advantages of backwards compatibility. It's a tradeoff, there is no universally right solution there. If you want to write code which is meant to last for decades unchanged you want ISO C or C++, backwards compatibility is a primary virtue there. My ANSI C89 code from 20 years ago still builds and works as expected when compiled as ISO C11 with the newest compilers. Of course there are also languages between these extremes.