The all stainless Sig weighs the same as an all steel 1911. People carry and have carried all steel pistols and revolvers for decades with no problem. It would not phase me one bit, but thats just me.
The aluminum framed Sig 226 passed the super demanding military trials in the early eighties as did the Beretta 92FS (the most demanding test in pistol history). According to the results with the longgevity of the frames, the regular alloy frame Sig will out last you unless you are a heavy competition shooter with a deep ammo wallet. And then it will still last years & years. I have a Beretta 92fs made in 1992 that will probably outlast me. If you keep the recoil spring changed every two or three thousand rounds, it will last a lifetime.
These pistols were designed from the start with the aluminum alloy in mind and thus the engineers knew the strengths of the materials. They are beefed up in the areas that they need to be. Most issues with aluminum are with steel framed guns that are made into lightweight models such as the 1911s. They still have the thinner slide to frame rails and metal thickness' like as if they were made of steel. That is where going alloy causes dissappointment. Next time you look at a Sig, look at the size/thickness of the rails and thickness of the dustcover and compare that to an all steel 1911 or CZ. It is much beefier and desined to last just as long.
Now with all of this said, the all stainless Sig will outlast them all because it is waay over engineered for being made out of steel since it is an aluminum frame design.