In many situations, the work is not indivisible. If AI can handle 80% of the work, then a company can let AI handle that 80%, fire 80% of their people, and consolidate the remaining 20% still-human-work with whoever is left.

That all being said I strongly disagree that GPT-5 isn't categorically better, I just think it's less obvious because we're starting to hit human cognitive ability to even assess that limits.