Maybe no indictment until he leaves office, but then there is the matter of state crimes. Trump will be tried and found guilty of state tax fraud and money laundering at the least. Even if he is pardoned by himself or some other President, that does not absolve him of state crimes. For that matter, accepting a pardon means you plead guilty to the crime, so the state version of those crimes will be slam-dunk cases.

If we don't prosecute treason, then what are we telling the next guy who thinks about committing treason?

I also think you are ignoring the power of the House committees to subpoena Mueller and learn everything he knows.