Ahem.... Last game with group number one saw a fight with a shadow dragon. The group is 11th level, 6 characters strong. My players are optimizers through and through (in those group) with 20 to 40 years playing with me. That little fight lasted... 24 rounds. Yep 24 rounds. And the dragon is still alive but fled. 3 characters died, one died because I used magic missile to kill him so that the cleric could not revived him. So yep, low level spells can be of utmost importance even at high and ultra high levels. And this dragon was a standard dragon with spell.casting ability as an option. Let's just say that I caught them with their pants down. And that dragon will come back at them with minions next time. Unless they find its lair first...
Yes the game is based around the 6-8 encounters and fights lasting around 5 rounds on average. But this is an average. Some fights last less, others last a lot longer. And again, a monster can appear more than once and the spell slot system is much better to simulate resource attrition than the stat block that they are going for.
Why didnt the shadow dragon use a cantrip over magic missile? Or the claw bite combo.
If it is due to not having a ranged attack well that's a separate problem. Because it still means lower level slots aren't useful to anyone who have both melee and range attacks.