The warlock is an entirely different case than the others. My issue was spending all of your adventuring day's spells on more high level spell slots than you have. A warlock cannot do that. First, they are short rest recovery. And second all of their spells al already their max-slot -- what this would do would let them cast spells that are less than their full slot. So the Warlock with their Pact Magic isn't what I was talking about in even a single detail.
And frankly, if damage scaling is put in to make those lesser spells do more damage, the point of this thread, then that would still be OP as the warlock could be casting more spells than expected and their damage would be scaled up.
As Distracted DM said, the A5E Warlock doesn't have to use their highest level spell slot when casting. It is a good improvement over the 5E D&D Warlock, and works well in practice.
The point that I'm trying to make about spell points is: You can choose if you want to nova with spell slots up to your highest level (5th max for A5E Warlock), getting the additional damage by casting a higher level spell, typically, or up-casting a lower level spell. Or you go with a lower level spell slot and conserve spell points. So it's a resource trade-off, and you're not nerfed with the lower spell slots being not useful in comparison to cantrips at higher levels, as you can use those spell points to caster a higher level spell slot, or some utility spell outside of combat.
Looking at the
variant spell points 5E D&D rules, let's say you're a 10th level spell caster. With 64 spell points, you can cast nine 5th level spells per long rest. That's very good. If the concern is about them going nova and burning out of spell points, then limit it to a smaller amount per short rest, say a third of the above, so 21 spell points, which is: 3x 5th level; or 3x 4th level + 1x 3rd level; or 1x 4th level + 3x 3rd level; etc., depending on how you want to break it down.
Getting back to the original proposal from FrogReaver, I think the assumption
by the 5E D&D creators is that higher level spellcasters won't typically bother casting say their 1st and 2nd level spells in combat to do damage, if they have 4th and 5th level spells in a big fight. But they'll still want to use those spell slots in other ways such as with Absorb Elements, Divine Favor, Expeditious Retreat, Healing Word, Shield, or for utility purposes outside combat, e.g. Detect Magic.