This is the one example of "collusion" that is easy for the public to digest and understand: Trump's son falls all over himself in his eagerness to meet with Russians who say they have dirt on Hillary. Especially if Trump, Sr. was aware of the meeting and its apparent purpose beforehand, this is easy to package into a sound bite that makes things look very bad for the President. Especially since the party line had been this was a meeting "primarily" about adoptions.
Sure, campaigns conduct opposition research all the time, but do they do so by meeting with representatives of foreign powers, especially with those who are ostensibly our enemies?
If Mueller alleges collusion or conspiracy with respect to this meeting and Trump's knowledge of it, I predict the blowback may be catastrophic.