I have not suggested that no crimes have been committed by those in the Trump campaign. Certainly, crimes have been committed. Some have already gone to jail.
From the very beginning, folks understood that it would very difficult to find direct proof of a transaction between the campaign and Russians: you give me this and I'll give you that. It was thought that the campaign was much too incompetent to be part of a conspiracy to affect the election.
What would be required for what is called "collusion" is a memo or transcript that says that if you release the wikileak documents, Trump will revoke the sanctions if he is elected. Or, if Trump removes sanctions, then Putin will approve theTrump Tower after Trump is out of office (and lend him the money). In this sense, there may be no collusion.
The investigation is about much, much more than this. The first clause asks the SC to investigate any link or cooperation of those in the Trump campaign with efforts by the Russians to try to influence the US elections. BTW, nothing need be proven with regard to whether these efforts were successful. IMHO, Mueller has found lots of this, and more, related to other clauses empowering the SC. Obviously, the easy thing to prove is lying to the FBI or Congress. But, there clearly have been links and cooperation.
What we still don't know is what they are hiding. Surely, Jr didn't present all those incorrect documents and misstatements about meetings for no reason at all. Manafort added 20 YEARS to his jail time when he decided to stop his cooperation and hide something (in hopes of a pardon).
====
The BOTTOM LINE is that there is lots that we don't know. Perhaps we're done and only 15 or so campaign and transition folks were involved. And perhaps Trump and/or his family were involved, or not.
Even though it isn't listed as a crime, there are several court filings which details the crimes committed to keep secret several meetings between Russians and Trump campaign officials. That itself is documentation of collusion between the two. The fact that is isn't a crime does make me wonder why certain people keep asking for evidence of it, but the fact that it isn't a crime doesn't make it go away.