Possibly months ago.
Mueller has nothing.
Once realized the deep state hatched a plan to:
A) Announce fake indictments
B) As you say pounce on Trump with scripted scorn
C) Cap the outrage with a charge of treason from a "credible source"
All staged and Trump plowed right through it.
You are correct - Mueller has nothing. However, ending the investigation without a collusion charge will vindicate Trump and serve as the final humiliation for the disgraced media/Democrats. Therefore, I predict that it will continue through 2020 or until Trump finally fires him.