Mueller already has indictments and convictions
I suspect there may be an implicit meaning of "what if there is not evidence of criminal collusion (conspiracy) or obstruction (of justice and investigation or Congress)?"
It is similar if not the same as asking, did L H Oswald shoot Pres Kennedy etal?
I think there are a couple of approaches to situations like we have. The one I like in this circumstance (only because there are only a finite number of possibilities) is to start with the premise, the Trump campaign criminally colluded with Russians to win an election.
It is relatively easy to process the premise. Start with all the campaign staff. Start with all the easily known Russian interactions. Follow the evidence. Either the accumulated evidence will meet the minimum requirements for criminal prosecution (or impeachment charges) of criminal conspiracy or it will not. In this case, the evidence should be overwhelmingly compelling or it should be dropped. There is no benefit to presenting a weak case to anyone, the public, grand jury, or Congress.
If there is not enough evidence to convince an armadillo Mr Trump or people in his campaign criminally colluded with Russians to win an election, then, clinch your jaw and accept it, and move on. There are more political battles to fight which are far more important than personal vendettas against some of the slugs who have been elected by an ignorant electorate.
Find long lasting solutions to our problems. The Founders gave us a frame work. We need to fill in the blanks.