In DnD 5e progression, Cantrips become stronger on level 5, 11 and 17. But Cantrips in the system are meant as a "basic" attack for casters to use when they can't or don't want to use spells. So yes, the expectation is that in most fights you'll have to use a spell or two.
Putting all that aside though, Larian changed the cantrips in such a way that most of them are just as potent as the average hit from the more martial classes. So in the current state of the game, you should be fine skipping a long rest or two.