Hopefully someone will be able to answer a question I have about antiobiotic resistance. This isn't my field. My understanding is that every adaptation like re

Hopefully someone will be able to answer a question I have about antiobiotic resistance. This isn't my field. My understanding is that every adaptation like resistance to families of antibiotics has a "cost" to the organism. These are genes they need to carry. "Useless" genes tend to be disappear over time. As such, if we, say, stopped using pennicilin for 20 years, when we started using it again, it would be much more effective as those adaptations would slowly be lost. Is there any truth to this? Has there been any research done on this? If true, how long would it take for such adaptations to be lost? Nevermind the practical issues with implementing such a policy. I'm just interested in the bacterial evolution side of it.