Well, you're still going to have many varieties (genetic profiles). The big seed companies will sell 5-10 varieties per region. Regions depend on growing season

Well, you're still going to have many varieties (genetic profiles). The big seed companies will sell 5-10 varieties per region. Regions depend on growing season length, soil conditions, climate. Iowa has 7 regions, IIRC. If I had to guess in the whole country there are ~100+ regions. That means thousands of hybrids across the different seed companies across the country. So even if a pest knocked out one variety (which doesn't really happen), it isn't like the whole system is going to come crumbling down. They spend billions researching it and there are lots of rules regarding "refuge" to ensure that insects and diseases don't build up a tolerance to treatments. Usually when you hear about people worry about "the mono-culture" they are advocating that we plant more of a different types of plants (canola, sunflower, sorghum, wheat), not different varieties of corn/beans. Also, that they are all identical is bit of a misnomer. They are, but they are exceptionally strong, vigorous plants (google "hybrid vigor" or "heterosis").