I considered this. There is a limited number of functions that can produce RawFD (os.open(), os.dup(), etc), and this is a single source of file descriptors in an isolated program. File descriptors can be inherited by child processes. But there are other methods of interprocess communication, and depending of the serialization method the type of the file descriptor can be lost. So there is a possibility to break existing programs. The transition can be made smooth, with corresponding deprecation period if we decide that the final befit is larger than the transition cost.

In any case this is a large change. It should be discussed on the Python-ideas or Python-Dev mailing lists.