Disabling plugins was a good attempt at resolution. Did you also try switching to default Twenty* theme? Themes can cause issues just like plugins can. Additionally, caching can confuse any investigation. Before deciding an issue persists, disable or flush any caches active, both client and server side.
I see author fields on my site, so the lack is not due to core WP. They could be hidden with CSS, but the state of all inactive plugins and a default theme should reveal them if caching is not involved.
If the fields are still missing in such a state, your core WP may have been corrupted. You should the perform a manual update, even if it’s to the same version. The point is to refresh all files from a fresh download.
FWIW, the author field only lists users who have authoring capability, so how many users you have in total is irrelevant, other than a large number will slow down the author query some.
