Are you of the opinion that Trump can lawfully fire anyone at the DOJ for any reason? Including the purpose of obstructing justice? ANY REASON? If that is what you are saying ...
History says you are wrong. I refer you to something that is quaintly known as the "
Saturday Night Massacre."
In that episode, no less than three people lost their jobs in one night, at the direction of the president. Robert Bork, Solicitor General, became acting Attorney General for Richard Nixon and carried out Nixon's order to fire the special prosecutor, Archibald Cox.
Why did Nixon want Cox fired? Because Cox had obtained a subpoena ordering Nixon to hand over recently discovered Oval Office recordings. Nixon would claim the firing was for 'insubordination.' These recordings later proved not only had the president known about and participated in the cover-up, but it made it clear that Nixon fired all these people in order to keep the tapes, which he knew to be incriminating, from coming to light. That, is by definition, obstruction of justice.
A court later ruled, and was never reversed, that firing the prosecutor was illegal. In other words, firing someone in order to thwart an investigation is not within the purview of the president.
Furthermore, it was later revealed in Bork's posthumously published memoirs, that Nixon had held out an offer of nominating Bork to the USSC in order to get him to follow his firing directive.
No, the president cannot legally fire anyone for any reason.