5e already does that
Except they don't need to wait till mid & high levels. Like driving a new car off the lot it starts the moment they hit level
three. and continues to slow at level six eight & ten. You're suggesting a nerf to casters so you can address a hypothetical problem that only exists in a contrived whiteroom scenario that starts with a quantum spell list & any reason at all to prevent noncasters from using their skills rather than consuming a spell slot.
But that's the number of
slots, not the number of
spell known.
Let's talk about a level 7 wizard. Let's asume that he has found
some spells but not a ton (so, 1 per level). Level 7 is definitely "mid level" but it's where spellcasters start being pretty powerful so it's good point to look at. Let's also say he has 18 intelligence.
This wizard's spellbook will have 4 cantrips + 6 starting spells + 12 "level up" spells + 7 "found" spells, for a grand total of 29 spells. Each spell is a tool to fix a "problem" - some challenge or obstacle you can face in your adventures. Sure, the wizard can only prepare 11 of these... but 15 with cantrips. And they have ritual magic. Let's just assume they know 1 ritual per level, and this wizard now has NINETEEN TOOLS in his pockets to fix problems.
Let us make a spell list
Cantrips:
Ray of frost
Light
Minor illusion
Mage hand.
... maybe I should stop here? This loadout is already equal to some of Troika's! magic users! This PC can attack and slow down enemies, make light, make illusions, and pick up things at a distance. If you knew someone in real life with those powers, you would call them AN AWESOME WIZARD.
... but no, let us continue
Level 1
shield
Fog Cloud
disguise self
Ritual: detect magic
Level 2
invisibility
Suggestion
Misty step
Ritual
Level 3
fireball
dispel magic
Fly
Ritual Leomund's Tiny Hut
Ritual Water Breathing. (I decided to put 2 at level 3)
Level 4
wall of fire
Evard's Black Tentacle
Look at the number of things this guy can do! All the problems he can solve! He can fly, deal with magic, protect the camp at night, deal with water adventures, control areas and do damage, mess with the mind of people, pretend to be someone else, teleport short distances, protect himself with a magical shield. this isn't a "fire mage", this is a "do almost anything" mage. And there are still a few more spells in his books that are more "niche" but very useful to have on certain daies.
This isn't some kind of theoretical mage, this could be a real PC with a real spell list. Edit: it's probably not even the best spell list by far.