They miss the most important facet. Costco can only make so many chickens in their rotisserie. The downside is bounded.
The best bang for your buck options at a buffet still come in a bucket, but there no obligation for the restaurant to keep that bucket full, regardless of demand. It's a bit like playing poker in vegas. You're competing with the other gamblers, not the house.
If a buffet consistently runs out of items they wont be popular for long. However, I think you've stumbled on an interesting point. I wonder if they could squeeze more profit by charging 2 rates: "prime time" and "after hours". If the buffet was $30 at prime-time, you would be promised that nothing is run-out. Let's say that window runs from 11:45am - 1:15pm. Then sell economy-class half-price tickets that are still AYCE, but some of the good stuff might be picked clean or have limited quantity.
This comes with the assumptions that 1) food waste should be minimized (i.e. can't be sold the next day, or less ideal to serve it at dinner hour if it was cooked for lunch), 2) the $15 customer would almost never pay $30, or very little overlap.
The best bang for your buck options at a buffet still come in a bucket, but there no obligation for the restaurant to keep that bucket full, regardless of demand. It's a bit like playing poker in vegas. You're competing with the other gamblers, not the house.