Pareto principal. We've seen the 80% but that last 20% is going to be really tough (and expensive). GPT5 illustrates this - it wasn't really better than GPT4o and in some ways worse.
AI doesn't have to replace human jobs as they exist today 1:1.
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.
Regardless you have no way of knowing where you are on the curve at any given point.
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.
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.