There's no reason they'd have to be down for two weeks in that case. They could use a binary distribution to immediately get back into shape, and then switch over after compilation's finished, or they could compile without -OX and it'd take almost nothing, and then they'd be good to go.

Really, though, nowadays compiling everything, even compiling things for optimal performance, doesn't take that long. Your production machines shouldn't have web browsers, and without the C++ garbage behemoths on it, compilation takes like half an hour on most machines.

I'm not a Gentoo user, but it's kind of silly to act like it makes recovering substantially harder than any other distribution.