mellored
Legend
I'm not sure "can cast high level spells" was ever in any design document of a warlock.Here's my metric: if a class receives spell levels at the same rate as a full caster (we can use the wizard as the standard example) and gets comparable spell slot value as a full caster, that class is a full caster.
"Had at will magic" was the original idea. (Now eveyone has at-will cantrips).
So really. Removing spell casting all together, and increasing the number of invocations would be closer to the "real" warlock.