You need to apply more critical thinking and consider the actual motivations prosecutors are under. It does no good to just model your adversaries as infinitely powerful demons and assume they have the will and capacity to do all the evil all the time. It makes you incapable of rational thought.

There are very real downsides to a prosecutor bringing a case again and again. Downsides that actually hurt their goals.

• With each trial the evidence gets less fresh.
• With each trial the prosecutor seems to more of the public as the villain.
• With enough trials defendants can start making legal claims based on due process grounds (not double jeopardy grounds.)
• Every trial is a roll of the dice that the jury will return a full verdict of not guilty, rather than merely a hung jury.
• With each trial the popularity of the defendant increases, upping political pressure on the prosecutor.
• After enough trials, the governor will be under pressure to just grant the defendant a pardon.
• With more and more trials, the prosecutor risks completely destroying their career. They risk being seen as an incompetent clown who either can't get a conviction or is too dumb to realize when he doesn't have a case.

You can assume the prosecutor is evil if you want. But that doesn't mean you can pretend that the prosecutors still aren't mortal human beings with the same worldly pressures as anyone else. And despite Trump's verbal diarrhea, federal prosecutors have had to abandon countless cases due to lack of evidence, Trump's tantrums be damned.

Even if you assume the prosecutor is willing to do anything to get the maximum conviction possible, they still will not endlessly try defendants. Each trial makes a conviction more unlikely. Which is why charges get watered down and plea offers get more generous.