D&D 5E Playing non-healer clerics

[ECMO3]

Refusing to use a class ability out of preference when that class ability would be useful to the group is uncooperative. It's a question of principle.

You can't have it both ways. Up above you said you agreed that telling a cleric in session 0 that he had to cast healing spells was uncooperative, and here you are saying if he doesn't then he is uncooperative.

If a player approaches D&D from the perspective that their character concept is more important than the enjoyment of the group, they've missed the point of the entire game.

EXACTLY. Your character concept of a cleric needing to heal should not be as important as the enjoyment of the group!

Demanding Clerics heal is the problem because forcing the Cleric to heal even when it will factually cause the game to be less enjoyable for the playersis contrary to the point of the game.