Single founders introduce a few other common failure modes that are alleviated by having cofounders: giving up, mental health, stamina, running out of money, and missing the market opportunity. It's like you can trade one big point of failure - cofounder dysfunction & breakups - for death by thousand paper cuts.

(Note also that institutional stakeholders would usually rather have a SPOF that they can identify and intervene in, rather than a bunch of little failure points that might sneak up on you and are very difficult to stave off. This may or may not be in the interests of the individual founder, who often has an information advantage and better mitigation strategies for identifying and counteracting these other failure points.)

This is a good point. Single founders fail when the single founder has problems. Relationships fail when both individuals have problems.

The value of a co-founder is that they can cover for some of your problems. But likewise, a co-founder can also introduce problems, and if their problems interact with your problems, then the relationship turns sour and you suffer a divorce, which can be its own problem, and tear the company apart.

I think there's a form of survivor bias (or non-survivor bias) going on here.

A key route to failure is making excuses. A bunch of people who failed telling you it's their cofounder's fault shouldn't be too big of a surprise.

Even if both founders agree it was Founder B's fault, that doesn't mean its true. It really depends on the personalities of the people involved. People take blame for things that are someone else's fault all the time. Especially if the other person is good at projecting or deflecting.

Interestingly in the latest StartX session there are more single-founder companies than not. Not sure if it the first time this happens, but it definitely didn't used to be like that.

if you want to fast, go alone. if you want to go far, go with others.