So let's then define "lifetime". Life time of hard combat? Life time of range trips? Life time of carbine classes? Life time of safe queen? Will you actually wear out a barrel/upper before that lesser lower?
Not available any more? How many 80% lowers are floating around out there? Lots of AK stuff is in "kit" form. With the proliferation of AR parts vendors, just how much AR stuff is floating around out in the wild? My guess is that there will be plenty of parts for a very looonnng time to come.
The beefier lowers may well last longer in hard and constant use but realistically, who really is living that life? For the cost of one beefier lower, average Joe can get 2-3 of the others. Now let's revisit that "life time" idea with that in mind. There is no discussion on accuracy since that's a function of the upper. Point is, you can buy whatever you like. Some folks buy a seriously bad a$$ 4x4 that never sees any harder terrain than the mall. Just being devils advocate here since the vast, vast majority of shooters will never, ever gain any advantage with the beefier lower other than a status symbol.
Milspec- again, is a minimum set of parameters to which the lowest bidder has to adhere to for gvt contracts.