Unless there is some body of experts or some organization with the authority to mandate standards, then the standards you speak of are actually set by the industry.
Case in point: how many times have you bought a car without A/C? How about ugly tire rims with hub caps? Over time, the auto industry has made A/C and alloy rimmed tires standard. They used to be options you paid extra for.
As far as firearms and magazines go, the Army can set their standards, the AF can have a different one, and LE can have something else. These standards are independent of the civilian (commercial) market.
So, unless someone creates a state level, national level, or worldwide level standards board for firearms manufacturing, the industry will continue to set the standards based on cost, demand, and legal restrictions.
Between 1994-2004, we saw restrictions on AW and HiCap Mags, which did nothing to the rate of gun violence. Therefore, the gov't allowed the AWB to expire. Those types of government restrictions impacted the standards the manufacturers could set for themselves for that period. It had nothing to do with military, LE or civilian demand, nor was it based on practical funtionality. They adopted a set of standards for Mil/LE, and a second set of standards for civilian products, conforming to the new firearms law.
My point is, standards can exist for a variety of reasons, and they can change suddenly and frequently. Getting caught up on the phrase "standard capacity magazine" as an argument is tenuous, since the industry standard will change to 10 rounds for all civilians if the law requires it.