> The change to sys.platform=='linux' would break code even on current platforms.

Correct. Compared to introducing 'linux3', I consider this the better
change - it likely breaks earlier (i.e. when porting to Python 3.3).

> OTOH, we have sys.platform=='win32' even on Windows 64bit; would this
> favor keeping 'linux2' on all versions of Linux as well?

While this has better compatibility, it's also a constant source of
irritation. Introducing 'win64' would have been a worse choice (just
as introducing 'linux3' would: incompatibility for no gain, since
the distinction between win32 and win64, from a Python POV, is
irrelevant). Plus, Microsoft dislikes the term Win64 somewhat, and
rather wants people to refer to the "Windows API".

I personally disliked 'linux2' when it was introduced, for its
incompatibilities. Anticipating that, some day, we may have 'Linux 4',
and so on, I still claim it is better to fix this now. We could even
come up with a 2to3 fixer for people who dual-source their code.