The idea is that PYTHON3PATH will be honored in preference to
PYTHONPATH, but PYTHONPATH will still be honored.  It's exposed only to
people who specifically need it.

However, I think it's misleading to call the Python 3 transition a
"transient" problem.  If we're lucky, it's going to be a decade of slow
progress.  If we're not, it will never end.  Try, for example, compiling
your system C library without "gets", and see what happens.  (And that's
been an issue for more than 10 years!)

The biggest way to shorten this problem is to provide tools and idioms
for developers to use when porting.  Specifically like this feature.

As far as "version specific file extensions" - I'd be very happy with
that but Guido has said several times that he doesn't like it.  Feel
free to discuss that with him, but this change is far less invasive. 
Users who don't need it will simply never know about it.