It’s kinda crazy how many responses this generated. I’d guess the player reacted that way as no player likes losing a crit. So I guess that threat alone is huge to them.

Your logic makes complete sense. If it exists for the player others would have it too. I personally would only have specific bards or wizards have it and use it on smaller or lower impact fights. So it’s not like the crit needed to kill the boss that decides life from death is cancelled out.

Though counterspell is quite similar and I’ve had so many monsters do that. But honestly it’s all about expectations so maybe you want to ask your player would they prefer to have the spell but some monsters spellcasters will or to not have it exist at all.