FrankenCub
.338 Win Mag
The group of guys I shoot with, and countless others I've talked to, have zero problems with polymer mags. And someone correct me if I'm wrong, but most of the armed forces are now using P-Mags because they are more reliable than the standard issue mags. Many service members were having family send them P-Mags while were deployed because they had constant problems with what Uncle Sam wanted them to use. I would suspect there's a reason your uppers wont function with polymer mags. Your uppers needing to use metal mags isn't the standard, I would bet it's more rare than you're willing to see.I have three uppers in three different calibers. NONE of them can use polymer mags. It absolutely IS as bad as 'you' think. Most mags I have, you need some sort of tool to get the baseplate off even when the mag is empty and not attached to the gun.
My point of defending fixed mags is due to ergonomics. Some of you bash the livin shit out of a rifle that can't swap mags because you guys think the other options are so much better. For many of us, me included, the AR platform is about ergonomics. It's extremely comfortable to shoot from virtually any position or angle. And it's mostly because of the pistol grip. I hate traditional rifle stocks, truly hate them. They aren't comfortable in the least to me. I busted my wrist up when I was a kid so that may have a lot to do with it. So instead of shooting guns that don't feel comfortable, I shoot handguns. Well, the AR makes it fun for me to shoot it. I have a shotgun that collects dust, I have a bolt gun that rarely gets shot, and a .22 semi that has also spent most of the time I've owned it collecting dust. At some point, all of those will get a pistol grip stock, even if I have to make it.