Plasmids are only one of the ways that bacteria adapt.
Adapting antibiotic resistance can take as little as a single nucleotide mutation in the right part of a bacterial genome. Many of the known antibiotic resistance mechanisms involve small mutations to critical proteins.
"In nature, plasmids carry genes that may benefit survival of the organism (e.g. antibiotic resistance),"
Plasmids cost energy, they are dropped in the course of natural selection if they stop conferring an advantage.
More quickly than you might think, too.
If we stop using antibiotics, the bacteria will become less resistant.