That's going to be very situational. There were plenty of longer lasting buffs which could last multiple encounters. Buffs could certainly be the more efficient option in terms of total resources. Fireball will work, at best, for 1 encounter (and even then, likely won't end that encounter). But a buff can last multiple encounters, and some of them are quite potent.
I'm reminded of the time my monk was eaten by a roper. It was just myself and the cleric. He withdrew from the fight, cast all of his buffs on himself, and then solo'd that roper without breaking a sweat. Then got me rezzed (he wasn't high enough level to do it himself).
Certainly, debuff could change that equation, but so could any DM counters. Let's say that the party mage loves throwing fireballs. The DM could just design every encounter where there is a counter for fireball (such as a mage who readies an action to counter spell it). It's not functionally different from spamming debuffs in every encounter. In both cases you're specifically designing encounters to counter the PC caster.