I'm trying to think.
Offhand, I can't think of anything that is man-rated which is designed to spend more time in space before reentry than the Soyuz vehicles.
I've only read a little, but as I recall the fuel is corrosive and maybe some of the seals deteriorate, too. It's not a matter of "hardening" them.
And I'm sure someone will know more than I. Frankly, I'm a bit in awe of the Soyuz as it is.
Arf