I think it is an erroneous assumption to equate "home defense" with the need for a military grade weapon.
It is almost a certainty that a HD will never be fired in that role. If it is it will probably not be more than a few rounds. It will most likely live a sheltered life, seldom handled, and when handled done so with care.
A commercial gun of non-tested parts and "inferior" metals such as a Winchester 94 or Remington 700 bolt gun, a Mossberg shotgun, or even a 10/22 would serve perfectly adequately for most most people in a HD role.
Military grade weapons are tough due the abuse soldiers put them through under field conditions. They are the 24/7 tool of a trade. Being frequently handled, disassembled, reassembled, and many times maintained incorrectly. They are fired to high round counts and see plenty of rapid fire. This is a completely different set of needs beyond those for typical HD use.
Saying that, I don't promote buying crap. And a milspec gun should be more reliable and more durable than a common commercial grade gun. But everything in a trade off.
For example I shoot a lot and compete with my rifles. I like a RRA 2-stage NM trigger. Even if it may not be a good choice for military service, it doesn't mean that the increased performance in trade for decreased service life isn't a comprise that I shouldn't be willing to make. I have no qualms with using that trigger in a HD gun. If it losses its first stage I will put in a new one and continue to enjoy a great trigger break. No big deal.
I own 4 BCMs and a few RRAs. I have owned Colt, a couple of BMs, a CMMG, and about 14 or 15 RRAs. My "things are breaking bad" gun is a Gov profile barreled BCM, but it is last on list for general or match shooting. It just isn't near as accurate as my RRAs, and milspec AR triggers are just rediculous for anything other than duty use. Plus the generous chambers in the BCMs stretch my brass more than necessary.
Point is if I had to only have one AR, it wouldn't be a milspec gun... the reason is simple - it WILL be used to shoot recreationally, regularly, and will most likely NEVER be used in a defensive shooting. If it was, the chances of it failing catastrophically during the few rounds necessary at that very moment are almost nil. With those odds combined with the fact I take good care of my stuff I would be willing to risk a reliable nicer gun for HD v a milspec gun.
The right gun totally depends on the user's preferences and priorities.