No, at any given moment, there will be some new good posts that have just been published. You don't have to curate all to curate some.

Agreed partly, because for some timezones that are not very active, the good posts are usually only visible the next day.

A bigger issue in my mind is the fact that curation rewards, intended to reward manual and intelligent curation showing "proof of brain", can be reliably beaten by just automatically front-running vote selling bots.

Totally agreed. This and a lot of other problems are the typical "tragedy of the commons". They could be solved by removing the concept of a common reward pool and tying votes to actual transfer of currency.