I think this is a good question, because it's getting to the heart of why we draw lines in the sand in a fantasy rpg. We know that there have always been sacred cows in D&D: even going into 5E, Monte and Mearls are talking about retaining D&Dish elements to be inclusive and avoid an angry backlash. And we know that introducing elements like gunpowder guns into D&D is very contentious because it changes the baseline assumptions. Changes to campaign settings, like FR's Spellplague, can also make fans upset.
Yet it's also true that nothing is objectively right or wrong or realistic in D&D. It's just an imaginary fantasy. In a fantasy, anything could go, but it doesn't work like that. Many of us hold onto cherished class mechanics; otherwise it's not D&D. Many of us insist on having our elves and dragons and iconic spells; otherwise it's not D&D.
So it's almost as if fantasy is so up in the air, so capricious, that it's difficult to be on the same page when roleplaying in a shared story unless a) nobody really cares about a cohesive story, or b) you hold tightly onto your imaginary conceptions for a sense of stability and belonging and common reference points.
So when you have a vision of what a Paladin (or anything else in D&D) is, it can be one of those big anchors or reference points that helps keep the story cohesive. And if someone screws around with your reference points, it can create cognitive dissonance ("a discomfort caused by holding conflicting cognitions (e.g., ideas, beliefs, values, emotional reactions) simultaneously") ie., holding 2 competing versions of an archetypical iconic paladin in the same story.
This isn't a sole excuse or reasoning against allowing for flexiblity of class concepts, because there's obviously issues of game fun and practicality and social contract that needs to be addressed. This is more of my attempt at amateur psychology
