Vilifying proprietary magazines is a formula for killing innovation. Standardizing magazines makes sense, and the current AR and AICS standards have their usefulness. Does it annoy me that the proprietary magazines for my hunting rifles (Benelli and CZ) are expensive, out of stock, and now that one of the rifles is discontinued may never be available again? Yes, it's annoying. All I had to do was choose a different rifle, but I didn't because I liked the propositions my choices offered. I wouldn't sell them now because I still like what they offer more than any rifle with a standardized magazine. I guess for me, the magazine doesn't make that much difference unless it's actually bad.
How about the Mini-14? Proprietary magazine there with some aftermarket support that most internet forum pundits will claim do not work (not been my limited experience). Nobody has to buy the Mini-14, and while I certainly wouldn't see the rationale of a military doing it, they seem to be extraordinarily popular. Despite being overshadowed by the "American Rifle," an awful lot of people have a Mini and 50 years of being saddled with a proprietary magazine hasn't killed it.