Since we've established that there's no real reason to use 7075 in an AR, wouldn't there be other materials that would be cheaper and perfectly adequate? Could the receivers be made out of brass? Or polymer? I know that polymer lowers already exist, but surely with the recent advances in polymer an upper could be made too.

Or possibly use wood to build the receivers? Wood receivers with a polymer buffer tube and barrel. Surely an AR built using those materials would stand up to real world use and be cheaper and lighter than the 7075 aluminum and steel that the TDP thumping Colt enthusiasts would have us believe are necessary to build a proper AR.