I've beat the crap out of my Sig with lots and lots of ammo (and not used gently either). I think in most cases these days moving to the plastic is an upgrade in terms of durability with a bonus of less weight.
Ruger has moved to a plastic trigger guard on their 10/22 and tests have shown the trigger guard is FAR less likely to break on a dropping impact.
Check your Glocks, your M&P's and on and on with the use of plastics.
As well, consider just what the money saving would be for Sig to switch from a small, lightly machined guide rod to a plastic one? Do you think the saving would be significant enough for Sig to risk their good reputation? They did the move as an upgrade!