That itself is an evokutionary pressure. Whatever allowed them to get missed would get amplified by reproduction. Only if it's entirely utterly and only random chance and not something like the method doesn't target certain cells would it not be applying pressure
You're viewing the cows in isolation here. Replace the cows with cancer cells with different mutations and it doesn't work anymore.
There's research going with certain types of cancer where the doses of cancer-killing drugs isn't intended to eradicate all cancer cells, but to keep it at a level that's manageable with future treatments.
The theory being that if you go too far, cells that are left with a "lucky" mutation can grow without competition, leaving the patient in an even-less treatable state down the line.
you're talking about cows in a HIV discussion, so I could have said "cows and napalm? we're talking about HIV!", but I'm charitably engaging with your analogy to show that evolutionary pressure is an active research area in disease treatment.
But there's a chance that any survivors learn to hide from planes and you've now given an evolutionary pressure to cows to hide when hearing airplanes or helicopters. Along with that a pressure that makes the ones that blend in less likely to be spotted to be bombed. So yes, you're right you probably won't get cows that can survive napalm itself but now you've got cows with camouflage and an instinct to hide.
