sorry, this is absolutely awful logic.
Damage is what you want in literally any fight, it doesn't matter if it kills the enemies or not.
The reality is that a lot of fights in DnD will be up against 5+ enemies (usually more in my experience).
A fireball on top of all of them let's say just 5, and it's an average fireball doing 20 or so damage. that's 100 damage for a third level slot.
It gets a lot more insane than that though. Sometimes you have a huge mob of people, our evocation wizard dropped a 4th level fireball in a room full of enemies we walked into when infiltrating a cult and did 365 damage in total. It's useful for almost every fight you'll ever be in.