I think once the neutral hide is introduced, culling will be less crucial, because everyone’s visible pool will be self-tailored. The overall photo pool size need only be restricted by site limitations.
Culling is still a good idea, though, to keep things tidy. Couple of thoughts:
Zero upvotes after a year would more than likely meet the poor ratio criterion anyway, and it wouldn’t auto-cull any captions that had been upvoted (and not downvoted obviously).
Ratio-based culling would work, but if a number of people see a photo and go ‘meh’ and neutral vote, this could meet the criteria for culling before others who may have liked the pic have even seen it.
Perhaps a hybrid of the 2 criteria would be best? i.e Only photos that have been in the pool for a year (or suitable length of time) are subject to ratio culling?
Alternatively, there’s always the 2 year lifespan option, which would cull pics by natural deselection.