Lacking slots doesn't exempt one from attrition -- in fact, a lot of the resourceless characters do poorly at endurance challenges (e.g.if you replace your party's Rune Knight with a Champion, you usually go fewer encounters before the party needs to rest or fail, rather than more. Same goes for replacing your Arcane Trickster or Soulknife with a Mastermind or something).

This is because the primary cause of attrition isn't using up ability slots, it's overcoming obstacles. Even a 'resourceless' character is spending check opportunities, action economy, hit points, or time.