A few counterpoints:
"Monoculture isn't new, and isn't driven by corporate megafarms. Monoculture is as old as human civilization."
Sure, individual farms have planted monocultures for thousands of years. But different farmers would often plant different strains. So in one area you'd have a couple of different cultivars, whereas now all of those farms are owned by the same entities and are typically planting exactly the same thing. As the farms get bigger and more uniform, what they plant gets more uniform too.
This is driven by a bunch of different factors, but one way or another nearly all of them can be traced back to the drive to optimize efficiency of production over everything else.
"Almost all mass produced crops are F1 hybrids. "
Yep. But hybrids of a relatively small set of parents. And that's the problem. They're all either the exact same strain, or they're related strains.
Compare, eg, against a "heritage" seed catalog. You get hundreds of strains. We used to grow those. The genetic diversity was higher then.
In Mexico, the historical record doesn't show hundreds of strains. It shows thousands. Thousands upon thousands. There is corn that's optimized to grow in upland deserts, and in salty soil, and in areas with a much shorter growing period - less than 60 days.
"I don't know when you think this natural robustness golden age was, but crop failures have been around as long as human civilization and we are now more prepared and resilient to these things than any time before, not less, because there are a large number of people with a lot of interest in designing the plants we use to survive to be able to respond to a variety of conditions."
It is absolutely true that as farming has become more scientific, we've gotten smarter about how to deal with pathogens after the fact. We have much better chemical controls now, and we have better biosecurity procedures to prevent diseases from spreading.
But.
You seem to be taking the stance that because we're better at dealing with one aspect of crop pathogens, therefore we're better at dealing with crop pathogens, period. I disagree. Just as with human diseases, it's entirely possible to on one hand have much better medicines available but on the other hand still be more susceptible to epidemics due to other factors that have gotten worse - antibiotic resistance, global travel, etc.
In the case of crop pathogens, we're more susceptible than we otherwise would have been because the genetic diversity of our major crop systems is nearly nonexistent. This is why seed banks are a thing - because ag scientists realize that at some point we'll run into a pathogen for which none of our chemical treatments are effective, and genetic resistance is our only chance -- and therefore we must preserve the genes of as many different varieties as possible.
This is what is happening with bananas, and possibly with cacao as well (black pod disease)... and citrus (citrus greening)... and maybe even wheat (stem rust). The point being, chemical responses to crop pathogens often aren't sufficient. Genetic modification might be, but personally I'd rather not place all my eggs in that basket.
For those who don't know the story: we used to all eat Gros Michel bananas. You can't buy a Gros Michel banana anymore; fusarium nearly wiped out the entire cultivar. We didn't, and still don't, have an effective chemical response to fusarium. So we switched to Cavendish, which was resistant. Now a new strain of fusarium is killing Cavendish, and it isn't clear what we're going to switch to next.
https://en.wikipedia.org/wiki/Panama_disease
Bananas are an extreme case because bananas are grown through root cuttings, ie they're all genetically identical, as you said above. Corn isn't as genetically fragile as bananas. But it's heading there.