What is the purpose of a Warlock, past level 9?
So, basically, this is what I'm seeing here...
The trade-off of the Warlock is that they have less spells they can cast in a day, but make up for it with powerful spells, since all of the spells they have are cast at max level...
...except, that's not actually true, because a Warlock's spell slots stop advancing at 5th level. They dob't have level 6, 7, 8, or 9 spells. They have "Arcanums" which can be used once per long rest and don't use spell slots.
So the problem here is this: a Warlock with The Fiend as his patron can cast Fireball. Since this is always cast at his highest spell slot level, it will start as an 8d6 Fireball per usual, increase to 9d6, then to 10d6.... and then stop forever, while a Wizard's Fireballs will continue on all the way up to 14d6.
So the moment you're a level 9 Warlock, you're already the strongest you'll ever be, and the moment you hit level 11, you're officially the weakest spellcasting class in the game. You have both less spells AND less powerful spells, and your other abilities don't make up for it. (IE. They don't actually have the martial ability of other hybrid caster classes, like the Paladin. They're about as good as a Cleric, which ALSO gets more spells and more powerful spells than they do.)
Is there an errata I'm missing somewhere?
EDIT: So, I thought of this as a solution... since capping at level 10 is stupid, but a level 20 Warlock getting to cast 4 level 9 spells every time they take a short rest would also be broken....
New Ability: Arcane Empowerment -- Starting at level 11, you can empower one spell that you cast, so that it is cast at the level of your highest level Mystic Arcanum, instead of the level of your highest spell slot. Once you use this ability, you can not use it again until you've finished a long rest.
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