This upper/lower combo solves more problems than the M10x. People will happily throw their money to who ever solves more problems.
Another thing about this upper/lower set is that MacCabbee Defense could have easily made a whole NR rifle and sold it at $2,###.xx (people would have lost their minds if that happened), but they chose to let us make whatever we wanted, with what ever budget. Unlike BCL, where you pay $1,750 and more $$$ just to get it to run right/reliably, this upper/lower combo is a win with the end user in mind. You can build this rifle between $1,800-3,000, depending on what you want.
M10x is completely proprietary, other than the magazine, brake, grip & trigger. M10x is regarded as the "modern lego build your own type gun" of the AK world where you don't have to deal with rivets and reverse engineering 1 time build techniques.
The SLR is essentially an AR, where you have mountains of parts at competitive prices opening a world of build your own lego gun. Anyone who is gonna run these guns past 10,000 rounds will be happier choosing an AR over the M10x.
My 0.02