I think that Trump hasn't fired Sessions ( the incompetent, worse than useless POS ) because there are quite a few GOPE types who would happily impeach Trump for doing so.
That is what is really happening. Look what the GOPe did to keep Thad Cochran in the Senate.
Sessions, Mueller and Rosenstein will get the boot after the midterms.
I don't think any Republican Senators would be stupid enough to go along with that. The real reason Trump hasn't fired Sessions is that his replacement would have to be approved by the Senate. Imagine the complete circus that would be. Trump is better off waiting to see exactly what he needs to react to before he reacts.
You have to have grounds for impeachment. There are none! Firing an AG is the prerogative of the president.