If he fires Sessions, the Senate will balk on any appointments; thus, the DOJ power will return to the Obama hold-overs.
The Prez is sort of between a rock and the hard place, at least until after the mid-terms. Then, he might gain a couple more (R) senators to improve his chances of moving his agenda and appointees.
Between the recuses and buck-passing, Sessions has been a disappointment. Some think this is a facade and ‘things’ are going on behind the scenes. So far, however, there is little evidence of that.
Actually Trump could appoint another cabinet member approved by the Senate as AG without going through another hearing. Dems and press would go ballistic but he could legally. Sessions just needs to say he has health problems, wants to spend more time with family and grandkids, anything of that nature.