I suspect that the idea originally was to give some safety valve but if it is used more than a few times by a President, it makes a mockery of it and it should be removed as a power. How can a President ever decide that the entire legal process is flawed and their opinion is right? If the sentence was too long then change the sentencing guidelines.
Don’t dress up your stance in fancy garb when it comes down to something baser.
What you are doing here is a distraction from the topic - whataboutism.
Personally, I blame lawyers and prosecutors. A law should be simple, easy to evaluate if it was broken, and always prosecuted. And when it comes to punishments, they should be explicit and without the possibility of being altered.
We've gotten too complacent with making all these arbitrary rules, then fiddling with their non-enforcement and severity by virtue of reduced sentences.