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Bacteria both evolve resistance quickly, they also lose it just as quick. Because unless it directly is needed in the here and now, the bacteria that remove it and survive without it outcompete the rest of the bacteria.
This is my understanding at least, a real bacteriologist can comment what I inevitably got wrong. But the recent study that used existing antibiotics in a certain way to kill bacteria is a great demonstration of bacteria not being able to select for all simultaneously and retain the old resistance in the future.
I'm not overly concerned about bacterial resistance given there are ways we can work around it.
Alas, experiments have shown that it is possible for resistant strains to strongly reduce the cost of resistance maintenance. If there are no naive invaders, this can lead to an evolutionary stable situation where the maintenance of resistance is less costly than losing it.
See Lenski's (old) review "Bacterial evolution and the cost of antibiotic resistance"