I'm sorry, but the argument that multiclasses are a way to cherry pick optimial builds is kinda flawed in a time when we have dozen of mixed subclasses and feats on top of that that offer the same level of optimisation. Subclasses (and classes) are just arbitrary constructions. Paladin is basically a predefined fighter/cleric mix (and actually quite imbalanced compared to other classes), ranger a drudic/cleric mix, arcane trickster or eldritch knights - do I even need to mention those ones? Or Spell singer wizards? There is a reason why the subclasses are constantly expanded - because people want more differentiated options to play their characters.
Sometimes people prefer there own flavour and game style. The rules are just mechanics, but not the character, nor its story. If you can't separate those 2 things you are limiting your own expressions (something that the D&D ruleset actively incourages you to do, as most experienced players/DMs).
At the same time I don't mind single class builds, but whatever the player chooses should be based on what their character's story and personal playing preferences suit, not some arbitrary mechanics whose only actual purpose is to offer some balance between those available options. But definitely don't tell me multiclassing is cherry-picking/powergaming as long as official single class builds like the twilight cleric exist. You can powergame and optimise in both approaches and also avoid it.
