A few points are worth noting about this.
Inspectors General are not political appointees. And they are independent of political appointees. So Donald Trump or Jeff Sessions cannot just shut this down when they get into office. They can try of course. But the statutes governing IGs specifically prevent this.
The final result in the 2016 election was extraordinarily close. Less than a hundred thousand votes made the difference in Michigan, Wisconsin and Pennsylvania together. That's close enough that lots of things made the final difference. But there's a lot of evidence that Comey's letter changed the outcome of the election - evidence borne out by a close analysis of the public polls and the private polls of both campaigns. It is a big, big deal. It is important to be able to recognize this while similarly recognizing that this doesn't mean there aren't also problems with the Democratic party, Hillary Clinton as a candidate, etc etc. They are simply two different issues that deserve to be dealt with each on their own merits.
There have been a number of studies of the polls and more that make a strong case that Comey's letter made the difference. Here's another one that came out just yesterday.