For those who pooh pooh the Mueller Rpt, I suspect they have conflated a mostly uncorroborated Steele Dossier, no criminal charges having been filed with exoneration, and the proof is Steele .... no charges.
Collusion is not a legal construct, however when one sees the definition, we have
secret or illegal cooperation or conspiracy, especially in order to cheat or deceive others
Is this not what the Trump campaign was doing? Was it well documented by the Mueller Rpt? Then one should be able to reasonably conclude the Trump campaign colluded with the Russians. Was it an indictable legal offense? Nope, but it categorically characterizes the campaign as ... to use the legal term ... scumbags.
Had Trump not been the current occupant of the WH, he most likely would have been indicted for obstruction. I believe based on the Mueller Rpt, those indictments were sustainable.
edit: That is not to say Trump would have been convicted. I reasonably suspect any legal proceedings against The Don will result in acquittal by means of jury nullification. He has poisoned the very fabric of our country and Democracy.
We will be frakked for many years to come.