It’s true that Trump may fire Sessions at any time, for any reason, as Sessions serves at the pleasure of the President.
However, replacing Sessions would require Senate confirmation, unless it was a recess appointment.
Or Trump could do a Nixon and A) demand Sessions fire Mueller, B) fire Sessions if he doesn’t comply, C) demand the Deputy AG fire Mueller, D) fire him if he doesn’t comply, E) demand the next guy in line fire Mueller, F) etc. until he finds someone willing to do it.
What Trump fails to grasp is that either of these would be politically unwise. Just as Nixon’s Saturday Night Massacre was the beginning of the end for him, Trump’s obvious attempt to halt Mueller’s investigation will essentially paint a large “GUILTY!” sign across his forehead.