I think context is important here. Is quick healing actually plentiful? If you go with the notion that PCs are rare special creatures, then no most groups wouldn’t have access to such tools. Even if you recognize a “holy man” in the group, are they actually a spell caster?
now let’s say the cleric casts a healing spell, so we know that is an option. Changing tactics like that requires a certain amount of rigor…lowering your eyes against an enemy trying to kill you to stop an enemy that is not requires a certain amount of thinking that goes beyond what most people train for.
so in a nutshell, it’s narratively reasonable that enemies would not attempt to coup de grace in combat even with healing present. It’s also reasonable if you did decide they would do it in that kind of world. Aka both options are narratively reasonable