You raise a good argument here. What is the purpose of this investigation? Congress very much likes that Comey insinuated (but expressly stated the opposite) a crime might have been committed.
Congress, being 100% risk averse and 80% against Trump was pleased to get the hand up, and went along with the false premise, never stated premise, that a special counsel was necessary, "in order to remove the appearance of bias." Congress wants the Special Counsel to do the dirty work. Same happened in the Fiske/Starr endeavor.
But in order for this approach to be legitimate, there must be a credible allegation of a crime.
If that can't be done, then release the Special Counsel, and put the monkey on Congress.
-- The investigation should be done probably by the House IMO and not the DOJ because of prima facia conflict of interest. --
Congress is per se biased. It's function is to be partisan. Nobody in Congress is unbiased, by definition. But when the point of the investigation is to impeach and remove a president or other official for something that is not a crime, then by golly, it's theirs and theirs alone to have at.