I'm not disagreeing but to be fair, it wasn't just Mitch! It was Republicans like Ron johnson, Cruz, etc who wouldn't sign their blue slips back when Dems had the Senate so that appeals and circuit court judges seats were unfilled until trump took over, a norm that was not given to the democrats when the shoe was on the other foot.
These people are playing Calvinball with norms to achieve their ideological goal: an explicitly conservative tribunal of last resort with lifetime appointments so they can make the hard decisions of striking down popular but un-"conservative" positions like environmental regulations and laws(specifically but not exclusively), medicare and social security, the right to choose generally, protection for pre-existing conditions, etc.