There are a multitude of commercial corn varieties lots and lots of varieties and many to fill all the niches. Take AgriGold A636-16 and Dekalb DKC64-32, they both fill roughly the same niche and are untraited but still have different disease and agronomic tolerances. That's more like the holstein to jersey comparison (both of which are used in dairy btw). This is distinct from transgenics which is good because trait durability is a more difficult issue to address at the development level than a product failure.

A transgenic pest control mode of action can (and has) failed like the first glyphosate tolerance trait but that doesn't destroy a variety.