You're not addressing the central point of my comment: that it is the DM's right to allow or disallow certain things, and that your contention that it is equal to another player vetoing a class is an irrelevant comparison. Dark Sun clerics is absolutely the same thing, since it is all about the DM's vision for the campaign. I'm not particularly interested in getting into nuts and bolts power comparison, since that is dragging the conversation into territory that is irrelevant to my point, but instead to say that a DM's personal preference is absolutely a reason to disallow multiclassing.
The DM's personal preference for character choice is for his characters, not mine. My PCs are my choice.
The DM really does have a choice about the in-game things he allows in his game, but MCing is not in-game, it's metagame. The creatures in the campaign world have no idea about game mechanics directly, only about how those things might manifest within that world. So, clerics getting power from gods is a thing that can be observed in-game, but the direct game mechanics of class cannot be observed. So the creatures could not know that this cleric has, say, a level of fighter, just that this cleric is good at fighting. You can't reasonable disallow 'good at fighting' as an observable concept.
For the metagame, DMs can certainly allow or disallow things. Spells, spellcasting, armour types (no full plate in my
world thank you very much; the technology doesn't exist yet and I want a different flavour).
But you cannot say 'characters cannot have an Armour Class because I don't allow attack rolls to fail'.
You can say you don't allow warlocks, you can say you won't allow fighters, but if you allow those classes and the abilities that they have then you cannot disallow MC fighter/warlocks on the grounds that it doesn't fit the vision for your campaign. You already do allow those things in your campaign!
If you allow training in weapons and armour, and you allow warlocks, you cannot say that warlocks with weapon/armour training are against the laws of physics! What in-game reason prevents them?
There is no in-game reason why warlocks would be incapable of learning weapon skills. There is no metagame reason, because you already allow warlocks and fighters.
All you've got is that you
don't 'like' it, therefore no-one else is allowed it. You have emotion, but not reason. It is not a reasoned position.
If your world has different coloured paints and you allow shields to be painted, and you're okay with red shields and blue shields, but don't like shields that are half red and half blue, then that is a personal preference you are entitled to have. But the idea that there is something about the universe which prevents shields that are half red and half blue is absurd. In-game you can have a School of Heraldry which disallows that kind of mix, but there is no law of the universe which could prevent it, even if the College of Arms might have a word with the PC.
But red paint and blue paint exist in the world. Character classes do not exist in the world, they are metagame mechanics.
There is no MC character whose abilities you do not already allow, so disallowing a PC with those mechanics is against reason.
When 5E made MCing and feats 'optional', it was an aid to get newbies into the game quicker. Like stabilisers on your first bike, they are meant to be removed once the newbie has got the hang of it.
There is no other valid reason. "I don't like it so you can't have it" is a childish response not worthy of a DM.