I agree that there should be no check on the power to pardon. There can't be because Congress does have the power to convict in some rare cases. The check is on the legislative as well as on the judicial branch.
Or worse, had someone murder his political opponent and then pardoned him? This sort of thing was unthinkable a few years ago, but no longer.
https://avalon.law.yale.edu/18th_century/fed69.asp
> ...If a governor of New York, therefore, should be at the head of any such conspiracy, until the design had been ripened into actual hostility he could insure his accomplices and adherents an entire impunity...
District Attorneys, the FBI and other agency's responsible for prosecuting crimes report to the President as part of the executive. Each has broad discretionary power on which crimes they prosecute, what sentence they seek, and what deals get cut. The president having some authority to wave crimes goes along with this ( and also short-circuits the president manipulating law enforcement for their own purposes ).
If the president one day decided that he didn't want the executive branch to enforce the law and pardoned everyone... well they'd be impeached quickly, and the supreme court would almost certainly rule that the president doesn't have the power to pardon everyone. Assuredly an injunction would be issued to block the pardon coming into force.
An example: https://www.dcourier.com/news/2019/nov/10/veterans-day-dan-f...
Sometimes pardons are politically unpopular even when they are right or justly merciful. Pardoning someone who embarrassed the government, for instance -- I can easily imagine a majority of Congress choosing to deny Chelsea Manning's commutation.
It's a bit of a parallel to the "better a hundred guilty go free, than one innocent suffer" philosophy that is supposed to rule the court system -- I'd rather the pardon be flawed in being too generous than too stingy with its mercy.