There is one extremely compelling reason why you don't prosecute your former president for things he did as president: If you do, it WILL be done unto you, with compound interest. And then it escalates until you reach the point where the only way to survive is to simply never stop being president. There is enough of those nations in the world as is.
I would have heartily welcomed the initiative if you had a Republican administration now. But the fact that it could only happen under a Democratic administration is why it must not happen at all. It will be seen as petty vengeance by the losing side, and they may not be the losing side forever.