I wouldn't say the design is outdated. In fact it was developed in the late 80's and adopted around 1990. The latest designs offer adjustable collapsing stocks that fully fold. There are now lower receivers that take the M16FOW mags (which have come a long way but are not as good as the Swiss Arms mags). There are now charging handles that are straight and do not interfere with optics on the flat-top models.
Swiss Arms still enjoys sales to smaller specialized units in many Countries and they are very popular with Tactical Teams in Europe. They do not advertise their sales and you would be surprised at which forces, and how many, are using them.
The system offers a level of quality, workmanship and reliability that is without equal. Sig Sauer USA ramped up their clone of the original and I think it's not that bad of a gun but now where near the quality of the original Swiss made ones. I tested one with a 16" barrel and our SG552's with 9" barrels grouped better at 100 meters, I couldn't believe it!
They are so reliable, run wet, dry, dirty, clean, hot...really hot and just never quit. They feed any ammo including some of the obscure frangible stuff that doesn't even look like real bullets. The little guys, SG552 / SG553 need their extractors changed between 5 and 7 thousand rounds. This is on select fire guns with much higher firing schedules than the recreational shooter with 5 or 10 rounds and semi-auto guns. (Take a look at some of the parts failures with other short barrel 5.56mm guns, often there is a lot more going on with them.) Short gun, high pressure. The bigger guns, 551 and 550's don't loose their extractors and just go and go. The bolt lugs are massive and will easily out last other designs with multiple little lugs.
All this said, they are expensive. There are many other guns that work very very well and are cheaper. As one of the HK Engineers in Germany told me, "Of course the best gun is the Sig 550, but the price killed it, it's just too expensive for most uses.", we will leave him un-named at this time as he still works for HK. I too am a steady drinker of HK coolaid and Knights coolaid but the Swiss Arms I believe is better.
Rich