I see where you're going with that, but I don't see a sling and bullets as equivalent to a bow and arrow... The difference in damage between a short bow and sling is not that significant.
Yeah, like most analogies, it's not perfect, but I was thinking damage, range, magic ammo, etcetera. The longbow is superior in many ways.
And I'm not claiming that spells should be equivalent. The common example being used is burning hands versus fireball. Burning Hands cast as a 3rd level spell should be on par with Fireball.
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Otherwise, my experience is that my spell is wasted if I do not use that 3rd level slot to cast Fireball, even if Fireball is not the most strategic or useful option in situations that require things to get hurt/killed/damaged.
That's where we differ, even though I completely understand your position.
I think it's OK that a wizard's spells are a bit 'level specific', where stuff like Entangle becomes less and less useful as creatures get bigger and stronger, the go-to level-equivalent spells are different, and that means you have to adjust tactics rather than staying with the ones you used at lower level.
If every single low-level spell can be bumped to a higher slot to be *almost* as effective as the go-to spell of that higher level, then you basically give the caster an insane amount of flexibility.
For example, Flaming Hands isn't that powerful, and it's area of effect is fairly small and manageable - it can be cast in ways that don't endanger your party mates in a small to medium space. Fireball, however, is much more potent, but also comes with the limitation of being a HUGE area of effect that makes it more dangerous or even unusable in some situations. That seems like an innate feature in balance and flavor. That makes fights in different spaces have different characteristics and solutions, and IMO that makes it more interesting.
I also hated 3.x due to the power creep in casters, so I think it's additionally risky to change it based on common sense or 'fairness' when that's at risk. So, YMMV, and if someone home-brewed these in their game I wouldn't quit or anything, but when I DM I leave these as written. It hasn't been a problem yet.