>This is an important question because projects like Python will live that long.

Planning in the 20 or 50 years range is counter-productive.

Heck, whole countries might not exist by that time (the same way ex-USSR and Eastern European countries split into different ones).

It's a significant possibility that Github the company will die, but github the site will not, at least not immediately. For example, github could be bought, in which case there would be no immediate reason to think the site would cease to exist immediately. On the other hand, I find it hard to imagine that github the company could fail (i.e. die off) without github the site becoming extraordinarily less popular, in which case we shouldn't really care if github the site is still around. Of course there's also the option that code hosting becomes an inherently unprofitable business, in which case we're sort of screwed all around.

Trying to figure out which of these scenarios (or a different one!) is going to happen sure looks like optimizing prematurely to me, in particular so long as there does exist some fashion of a backup repository.

It's not impossible to move to a different platform.