You are making an important assumption, that some indictable form of collusion by Trump that would warrant impeachment would have swayed enough votes to cause the Democrat candidate to win the election instead of the Republican. However, there is no way to actually know or prove that…. and it’s a simplistic and not neccesarly accurate assumption to make.
In theory, it could be definitively proven whether or not Trump engaged in some sort of wrongdoing.
It CAN’T be proven whether that act of wrongdoing would have swayed enough votes to change the election results. There is no way to actually know that unless you are psychic.