There are downsides to accepting a pardon, including the waiver of his Fifth Amendment rights. Trump might be immune from federal prosecution for supposed crimes committed during or even well before his term in office, but he would still be legally obligated to answer questions in any future showboating hearings House Democrats choose to conduct on pain of charges for contempt. This is something he would probably prefer to avoid.
One also gets the impression that a pardon is exactly what his successor is hoping for. Instead of having to decide whether appeasing the Arlington dog moms chanting “Lock him up!” should be the first priority of his administration, Biden could simultaneously insist that he is taking the high road and lament his inability to direct the Justice Department to investigate Trump. All the dirty work could be left to state-level authorities, who have a vast range of tax and other relevant statutes at their disposal. As a lawyer friend in New York put it to me recently, “Trump is going to be holding remote campaign rallies in 2024 from either Dubai or Sing Sing.”