The only genuine reason I see for not investing in single founders is that startups are a lot of work and it's unreasonable to expect that a single founder can

The only genuine reason I see for not investing in single founders is that startups are a lot of work and it's unreasonable to expect that a single founder can focus on all the different bits that need to align for a business to take off. However, taking this as a given doesn't strike me as right as founders can definitely get to some sort of market traction on their own and hire people to fill in their deficiencies. It also strikes me as odd that while Dropbox is one of YC's biggest hits they still hang on to this 2-founders doctrine. I can't help but think that not investing in single founders is caused by the social signalling involved of single individuals. The reasoning would be "If you can't convince somebody else to join you, why would I invest my money in you?". Another genuine concern would be that this individual would have to have mastered a broad set of skills to run a successful business independently and such people are unicorns.