I'm from Iowa. I now live in central Pennsylvania. I have ridden a decent amount of Iowa gravel and I probably wouldn't see the need for disc brakes in Iowa either. Unless the gravel I rode was near Dubuque. Some people ride on rail trails and call that gravel. But here in Central PA, I have seen people with cantis smoking their brakes down a mountain because they were afraid they couldn't stop and I was really glad I have discs. I was a little upset when the industry decided they were going to focus on disc brakes, but for gravel, I think the majority of riders are better off. And the people from the midwestern U.S. have to compete for the dwindling supply of used Poprads or just accept disc brakes.
The two companies people always cite as all road with cantis, Black Mountain Cycles and Crust have introduced disc models. They both still have cantilever models. It wouldn't surprise me if those canti models went away because the demand is too small.