It does seem to be the convention though... Impeachment would seem the only real threat (depending how the mid terms go)
I think Trump's main fear (and I think it's also the tactic of Mueller/SDNY/Avenatti) is that he will be impeached due to campaign violations or obstruction of justice before the Russia probe reaches it's conclusion.
At that point, he's a private citizen with no presidential pardon powers and the potential charges he faces are no small change either. Forgetting the shallow conspiracy charges that will relate to the Trump Tower meeting, it's what they will have on him before that. In huge debt to Russia with quid pro quo agreements in place, it'll be the equivalent of a murder charge compared to the manslaughter charge of "so we got offered some stuff and we said ok why not?". This is a long time in the planning and if he's nailed for it as a private citizen, he will be well and truly fecked. I'm not convinced that pardoning him at that stage would be a good move politically either as he'll essentially be proven to be a treasonous snake.