But how long is that going to take? Resistance has very high fitness in the presence of humans using antibiotics (avoiding death); resistance has very small negative fitness in the absence of humans using antibiotics (saving a tiny bit on metabolism). However long it took to evolve resistance, it'll take many times longer to de-evolve it. And given randomness, a tremendous number of copies of the resistance genes will hang around anyway so the resistance will come back even faster the second time.