...but this conversation is fairly Wizard centric. This scaling damage debate does not include context of the various class spell lists, but perhaps more importantly does not take into account the Warlock spellcasting system (that automatically raises the spell slot) and Sorcerers (who know very few spells and likely would want their early level spell choices to be relevant later on in the game).
For me you raise a valid point because looking at it through a Warlock's eyes reveals additional concerns. For me, Warlock has always been narrow by design. Hit Fly and spam Eldritch Blast. It multi-targets and scales reasonably. You can meaningfully boost it with Agonising and Spear. So my first question is whether it is right to make Warlock broader? I could feel drawn to asserting something like "
No class should be narrow!" but then I consider a game that has all broad classes against another that has many broad classes plus some narrow ones. Wouldn't the latter offer more overall diversity of playing experiences? On the other hand, if I'm playing a Warlock because I like the Pacts and Invocations, but it's not my goal to play a narrow class: should I be punished for my choice? The ideal approach may be that all classes offer both narrow and broad options, but you can see that structurally that is going to be difficult to achieve. It's probably hugely more manageable to have some broad, some mixed, and some actually narrow.
However, if I wanted to broaden Warlock then one way to do that might be via an invocation at 5th level. This hypothetical invocation will trade off the over-powered spell options for better scaled ones just as @
Cap'n Kobold suggests. Through being Warlock-specific, it sidesteps the thorny problem of re-balancing all the other classes that have access to those same spells.
Elemental Bargain: Your 1st level and higher spells do more damage when you reach higher levels: they gain 1d6 at 5th level, 2d6 at 11th level and 3d6 at 17th level. However, your Fireball and Lightning Bolt
lose 3d6 from their base damage, e.g. they do 5d6+1d6 damage when you are 5th level.
If it turned out that some specific spell was broken by this, I might narrow it by using damage types i.e. only spells of some damage types get the benefit.