5e is still balanced because of the opportunity cost. If you grab that 1 level in warlock, you'll never get the level 20 perks of your main class. If you pick up that 1 warlock level when you're around level 5 it means you're postponing your next main class power spike by a whole level. The rewards have to be somewhat worth taking because of what you're losing.

And to be fair, none of the official campaigns even play at level 20. It may sound OP doing that much damage at will but you're missing the context. Your enemies at level 20 are basically going to be literal gods. You're going to be playing a homebrewed campaign. Your DM has many tools available to make your level 20 cantrip balanced, not least of which is just scaling up the enemy HP.