Maybe I can help change the direction of this topic a little:
What about leaving the upper more or less as-is and eliminating the buffer tube/buffer/spring, kinda like Olympic did with the OA93, but with the monolithic rail and interchangeable barrels? Then you could have a shorter overall length, and use side-folding stocks etc.
I don't know that it happens "all the time", but I've heard of problems with buffer-tubes bending when put under stress, or used as a club (don't ask me why a perfectly functioning gun is used as a club, but....). Once the tube is bent , the gun no longer works. Admittedly, the fixed-stock doesn't seem to have this problem, but both also can get sand/debris accumulation in the buffer-tube which can also be a source of malfunction. I am, by no means, an operator, and my guns get transported to and from shooting sessions in a climate-controlled vehicle, aren't used in the desert, and aren't used in any CQB stuff, so maybe all of these "problems" are striclty academic for me, but does anyone else consider this a weak point?