One other mechanism that would lead to the persistence of e.g. antibiotic resistance would be when the mutation that confers the resistance is not costly (e.g. a mutation which changes the shape of a protein targeted by an antibiotic to a different shape that, while equally functional, is not disrupted by the antibiotic). Note that I don’t actually know whether this mechanism is common in practice.
One other mechanism that would lead to the persistence of e.g. antibiotic resistance would be when the mutation that confers the resistance is not costly (e.g. a mutation which changes the shape of a protein targeted by an antibiotic to a different shape that, while equally functional, is not disrupted by the antibiotic). Note that I don’t actually know whether this mechanism is common in practice.