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

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.