>By prosecuting (former) politicians, you're not creating disincentives against abuse of power
Do you think prosecuting people who committed crimes creates a disincentive against crime?
>you're disincentivising loss of power.
The same could be said for (former) leaders of drug cartels and mafia bosses, but we still go after them (as well as the current ones), don't we?
Loss of power is already by default highly disincentivized, which is why people want to hold on to it for as long as they can. It's also why we enforce term limits and have checks and balances. However those checks and balances are quite toothless when you elevate politicians above the law.
"We hang the petty thieves and appoint the great ones to public office." -Aesop
Troops on the streets?
You're also misunderstanding the argument: It's not the powerful politicians that are frightening, it's the powerless. By prosecuting (former) politicians, you're not creating disincentives against abuse of power, you're disincentivising loss of power.
That's the hard-to-accept logic behind the mostly deferential treatment the Nixons and Mugabes of the world today get after they lost power.