I think we should differentiate between (i) single founders who can't find a cofounder and (ii) single founders who don't need a cofounder.
I think case (i) would be a red flag because it means the single founder sucks at selling herself, sucks at networking, or isn't resourceful enough to move to a location where there are more potential candidates.
In case (ii), where the single founder doesn't need a cofounder, that founder also probably doesn't need YC.
Well, lets look at that from the perspective of a possible co-founder: I can not find any founder that would resonate with my technological background, skill set and desired professionalism.
When you live in a startup hub, matching people for startups this might be an easier thing to accomplish, but in the in which I live right now..
I think case (i) would be a red flag because it means the single founder sucks at selling herself, sucks at networking, or isn't resourceful enough to move to a location where there are more potential candidates.
In case (ii), where the single founder doesn't need a cofounder, that founder also probably doesn't need YC.