Mueller's case against the President really can't be much weaker. Here is why:

 Mueller has indicted a total of 25 Russians individuals,  all living in Russia,  three Russian companies,  and five Americans (Gen. Flynn,  Manafort, and,  others).  Not one of these indictments has anything to do with so called Russian collusion with the secret agent, . . . . . 
Donald J Trump.  Nobody has "flipped."  You know what Mueller has,  at present?  Nothing.  
The axiom, " if you keep doing what you have always done,  you will get what you have always gotten  . . .  i.e. nothing."  Time for that axiom to be put into play;  time to end this embarrassment and move on.  
Question:  What happens if the Mueller prob last for three or four more years?  Well,  for one thing, there will be no action in the House as regards impeachment,  since the investigation that is to support impeachment is not finished.