It's much more complicated than you seem to think, otherwise there would be almost no non-resistant strains left.
For example, there typically seems to be a cost associated with maintaining resistance, and particularly maintaining multiple forms of resistance, and strains will often drop resistance absent constant selection pressure.
Adding onto this, I saw a very interesting seminar a few years ago from a biologist who sequenced bacterial samples from the arctic permafrost. He found resistance genes for antibiotics that had definitely not been created by humans at the time that the samples were deposited.
So "new" antibiotics aren't actually new, and resistance to them probably already exists somewhere -- bacteria have been fighting this war for as long as there have been bacteria.
For example, there typically seems to be a cost associated with maintaining resistance, and particularly maintaining multiple forms of resistance, and strains will often drop resistance absent constant selection pressure.