The entire point of the Stoner design isn't just DI - the barrel extension that locks the bolt directly to it, not the receiver, is what allows the forged aluminum upper and lower.
The forging process was chosen - circa the 1950's - as a high strength, mass volume method of construction. Hot forging aluminum platters with a set of dies allows a lot of parts to be built economically, and it significantly reduces machine time for the minimal cleanup left. Don't forget, these were carry handle uppers.
As time marched on and CNC became much cheaper, lo, the flat top A3 is introduced, with little significant increase in costs.
Consider: if all the receivers in an M16 need to do is keep the barrel extension connected to the rear stock, and house some moving parts, is forged aluminum really necessary? It's also been done with sheet metal stamping on other weapons, the HK91 goes back to the 1940's, too, another barrel extension design.
The ACR uses an extruded upper. Extrusion forms two axes of the final part in one pass, and done with precision dies, is a finished dimension. All that's left are the third axis cuts, easily done with todays CNC processes. Can that be done cheaper than forging, I have no doubt. In fact, I'm willing to put money into a business offering those - AR15 extruded aluminum monolithic uppers and lowers, I believe it's something Stoner would have jumped on if the technology had been more advanced and the company mission statement not focused on aeronautical technology.
A complete stripped set of upper and lower finished for $65 is entirely possible. If it can be CNC'd, it certainly can be extruded, and for less. I know it may not exist, I don't see any major impediment to a CNC extrusion process allowing some of the third axis features to be integrally formed - like the cam bolt bump out or fencing. Rails, not so much, they are precision.
Big picture, guns come and go, what most users miss is that technology will advance a concept and offer improvements on the design - sometimes enabling it decades later when production costs allow it. Case in point, it was common knowledge in 1940's physical science texts to discuss the characteristic of quartz vibration when an electric current passed thru it. Well, that took another 30 years to make into a common cheap watch.
The SCAR offers nothing in the way of a significant advance, just economical parts made the same way as a dozen other guns, and nothing new at all in he way of design. Nada. Hailing it as an advanced combat weapon when it doesn't advance antything new at all, and is based off 1950's firearms principles, is really asking us to take it on faith. There's obviously no science in claiming it's better - nobody can state any superior quality expressed in numbers as measured by a credible authority.
Something the proponents would insist be absolutely mandatory if they were plunking down $2000 for a 100 mpg carburetor. Yet they spout the same rhetoric as the ads proclaiming it the greatest invention in the history of man.