Actually, according to the various folks at S&W with whom I've discussed this from time to time, it costs more for S&W to work with Scandium alloyed aluminum frames, notwithstanding the expensive cost of Scandium in the first place. It would definitely be less costly to continue to use ordinary aircraft-grade aluminum alloy in their pistols and revolvers ...
As far as the wear? Well, I don't worry about it too much, since I've been carrying and using aluminum-framed S&W pistols for more than 16 years.
As long as the frame rails are properly lubricated they seem to hold up pretty well. The frame rails on a 6906 I used for a few years, and through which I fired an estimated 45,000+ rounds, didn't seem all that much more worn than on some of the other issued 5903 & 6906 pistols I've examined, carried by folks who only shot them when absolutely required to shoot them for qualification. I've seen some few folks fail to properly lubricate them, though, and that's certainly helped accelerate the expected wear & tear. I've also seen at least one regular aluminum frame which experienced some really worn & rounded rails, but the frame always seemed to be bone-dry whenever I had occasion to see it, too. Go figure.
I've only fired several hundred rounds through my fairly new SW1911SC 5", and the only unusual wear I've seen on the frame rails was where a short burred edge on the bottom of a slide rail had made a light groove on the top of one of the rails. I located the spot where the burred (steel) edge was located and dressed it.
I don't worry about wearing out an aluminum framed S&W pistol ... not with prudent and reasonable maintenance and their warranty and customer service. I'd worry more about the potential cost of the ammunition that might be required to wear one out.
I've come to really like my SW1911SC, BTW. Amazingly accurate ... even with all those MIM parts.
It easily equaled the accuracy of my Colt Government XSE, and then surpassed it.
I have yet to shoot a SW1911 that wasn't accurate, though, and I've shot a fair number of them.
I'm beginning to detect a theme ...