Why do scientists have to find a new antibiotic to kill bacteria?

After releasing enough of one type of antibiotic onto some bacteria, given enough time the bacteria will adapt, and even change to be unaffected by the given antibiotic. The only way to inhibit the bacteria then is find a new, better antibiotic, after which, given enough time and exposure, the bacteria will get used to this antibiotic, so there is again a need for a new one.