Having genetic diversity reduces chances of widespread plague when the pathogen ends up circumventing the resistance, but crops are often bred to have certain resistances to pathogens/insects, so if you risk having those resistances bred out. Additionally that variation in the f1 generation isn’t guaranteed to have any better genetics for dealing with the pathogen and risks having worse yield, worse tolerances to other environmental factors, etc. It’s like betting on Achilles or 100 untrained villagers, sure Achilles heel is a weakness, but he’s stronger and nearly invincible (hyperbole) compared to the mixed bag you’re getting with the villagers. You’re probably thinking of bananas being wiped out which is a bit of a different case, since wheat can be bred at a faster rate and is seasonal whereas banana trees aren’t as rapidly bred to deal with resistances/tolerances and the plants aren’t seasonal. If 30 of your 10,000 plants survive because they just so happened to have a better resistance by circumstance that you can’t guarantee you’re still at a massive loss, whereas if you go with the more productive line, you know you’ll get 15% more yield if there is no plague, but if there is then you might only have like 10 plants that survive.