This is just like when new Porsches come out. The purists insist that things must stay the same forever, refusing to recognize improvements. Eventually, they convert and bitch just as hard when the next model comes out.
This rifle is a big improvement over the Swiss rifle. It has the core of the 550, which was its big advantage over the AR (AK reliability and Swiss accuracy). But, the gun has been improved in all other aspects.
(1) Collapsing stock. Yes the old stock was pretty, but that shouldn't be the number one criteria in selecting a stock. The AR stock allows it to be adjusted to fit different users and body armor. Plus, this allows developments in AR design to quickly be incorporated into the SIG rifle. If SIG went totally proprietary, every time a new stock came out folks would complain that it wasn't available for the SIG (and probably claim they would buy a SIG but for).
(2) Rails. For better or worse, rails are the future. You may not feel the need to attach a lot of crap to your rifle (neither do I), but doing so is currently fashionable among military/police types and their emulators. SIGs target market is military and police, and that is one of the reasons civilians desire their products. Plus, after looking at those photos it seems likely the old furniture or a B&T item will fit.
(3) Lighter weight. SIG has shaved the weight of the rifle down into AR territory. This is important since, once again, you know that people would have whined endlessly about how it weighed more that the piece of crap they put together in the basement of their parent's house.
(4) AR mags. By switching they opened the rifle to all manner of magazines and accessories. C-mags, HK mags, etc. All are available. If SIG went with an exclusive design on this, everybody would immediately started bitching that they could get AR mags cheaper.
In other words, SIG has improved the 550. The rifle they are delivering is better in numerous ways. Frankly, almost all the "complaints" voiced by the nay-sayers come down to aesthetics. I understand that, looks are important in marketing a product. But it is clear to me that SIGARMs decided that, if they were going to make a rifle here, they were going to make it better, not just a clone of the original. Bravo SIG!