Remove that pressure, and the resistant population has a chance to rebound. More likely than having an ecology of other bacteria to fight with is that the field is somewhat or all but completely cleared. The body may not also have had a chance to clear up whatever condition allowed it to get a grip in the first place. Etc.
Again, as I understand it TB is a great example to look at, and depending on where you live it might even be a real danger to your and yours, so maybe read up on it?
Low doses allow you to provide a continuous challenge that doesn't kill the entire population. That's not what's happening when you stop your drug course early.
If it were true that resistant phenotypes often ARE dramatically less fit to the degree you imply we could simply stop taking Antibiotics and everything would revert back to it Susceptible state.
When you stop a dosage early you've very likely applying a low dosage to some subpopluation. Dosages are not linear, nor evenly applied throughout the entire population of bacteria in a human body.
As I learned as a rule of thumb while doing microbiological research in a NSSF Summer Science Training Program in 1977, most random antibiotics would fail to kill E. Coli at a rate of 1 in a million, and it went up to 1 in a billion with streptomycin (this is with random mutations, no plasmid acquisition involved, which is another story).
However, these surviving bugs are generally not as fit as wild type, they've probably survived by sacrificing something, or doing something that consumes more energy. This helps your immune system mop them up, and note if antibodies are involved (don't know about that) it takes a while to come up with new ones, plus if there's any sort of bacteria ecology in the target area that survives the antibiotic they'll out-compete them.
It shouldn't take much research on your part to confirm the basics of what I've related above, it's a tremendous problem, especially with TB, who's defenses require long and obnoxious drug schedules, resulting in more and more MDR and XDR (multi- and extensively-drug resistant) strains.
If you get XDR, you're probably going to die, in a era where that's squalid instead of romantic.
And I just came across "Totally drug-resistant tuberculosis" https://en.wikipedia.org/wiki/Totally_drug-resistant_tubercu... Not official yet, but....