Nothing says that the president is really, actually above the laws as his pardon power. You can see how it was, by design, a relic of an era of kings who were the law.
If presidential pardon can't be the last guardrail against injustice, that only tells me the justice system is not doing a good job and there need to be better safeguards that are built into the system. But then, we have a supreme court that has always argued, since the days of Rehnquist and maybe Scalia, as far as I remember, that being actually innocent is not good enough a reason to overturn an impending death penalty.
So yeah, we do have a justice system that is pretty badly f*cked up, but f*cking it up more by giving the president a king like authority that seats them above the courts is not a good path to go down on.
(There's a reason that at the end of every presidency we see pardons that don't make any sense and don't in any way serve justice. Biden seems to have entered that period of his presidential cycle.)
Also: I can make the same argument about governors. Their right to pardon needs to go too.