While that is correct (as I understand the rules), I also feel it's sort of lazy. I recently had a player ask if he could teleport farther if he cast Misty Step in a higher level slot (I allowed it). I think there are a lot of spells that, like Misty Step, could easily have expanded options for use in higher level slots. Longer range, longer duration, affecting more than a single target (for single-target spell), a larger AoE (for AoE spells), etc. are all good options for things that could potentially be done by casting any spell in a higher level slot.
Could have, yes. But I think it's intentional that they don't, in many cases - particularly with Concentration spells, or spells with tactical rather than immediate uses (like
misty step).
For example, take
misty step. As you say, it would be easy to expand it by saying that you could teleport longer for a higher-level spell slot. But if you look at the game in the larger sense, you'll see that there are few to no ways of getting more than about a single move out of a bonus action. Rogues, monks, and the spell
expeditious retreat let you Dash as a bonus action.
Misty step gives you 30 ft and have the advantage of not provoking opportunity attacks or actually crossing the intervening terrain. That's pretty much what I can recall off-hand as PC-available bonus move abilities. All of that means that if you want to move more than a double-move you need to spend your action doing it (for something like
dimension door), which in turn makes it more difficult to "kite" foes (outrunning them while attacking from a distance).
Take another example,
death ward. It would be trivially easy to give it an upgrade saying that for each higher-level spell slot, you could cast it on another creature. So why didn't they? Perhaps they
want the cleric to have to choose which party member to protect?
Haste could also have an upgrade giving it more targets, but that would lead to
haste being
expected in higher-level parties, and that's not good.