I think that this may be part of it. But I also think that the congressional Republicans believed that firing Sessions prior to the election would have hurt in the ballot box. Remember that those initial admonitions were early in the summer. It would have provided the MSM with a lot of ammunition to fling in Trump's direction.
Even now, they are talking about an Attorney General replacement AFTER the elections, but prior to the seating of the new Congress.
Good point as well.