The problem is that you're not able to do that. Even the most effective treatments do not kill all of the mites. So what you are doing is killing some of the most virulent mites, and leaving resistant mites alive. The next time you treat, the treatment is less effective and more of the most virulent mites survive. You're arming the most virulent mites with additional resistance, and at the same time, damaging the health of the bees. This is a treadmill that will just keep going faster until you can't keep up. As we've seen, acaricides begin to lose their effectiveness fairly rapidly; treatments that worked okay just a few years ago are no longer effective.
why should it be different with mites? what is so anti-selection about removing the most virulent mites from the mite gene pool (the one's that collapse a colony) while allowing the less virulent ones to coexist?