It's difficult to define "reasonable". What is reasonable for someone might be being able to speed run through a dungeon, while for another person reasonable might be simply being able to clear it.
Reasonable means just being able to clear it. Expecting speed runs out of everyone means you are not accounting for the fact that there are players new to content, and that is pretty unquestionably unreasonable even just on the basis that they were all newbs once too.

I can understand the frustration of not being able to finish a dungeon because one player is not pulling their own weight or geared appropriately, but to play devil's advocate, why is it that the healer should be the one to leave so you can finish the dungeon? I understand they were not skilled / geared well enough to heal everyone, but why does it mean they should be required to leave?
Because it's not fair (or respectful) for one incapable person to hold back three/seven capable ones?

Boy, that was easy.