I have always found the claim that single-founder startups are imperiled at odds with actually-existing reality; there have always been _tons_ of successful companies started and carried to fruition by highly motivated individuals. Individuals have been starting their own businesses since the very dawn of entrepreneurship. I don't know where the conventional wisdom that it's a bad idea ever came from. I see no basis for it other than in someone's thought experiment.
If you define a cofounder as someone who would continue the startup alone, two provide redundancy.
On the other hand, if you're overly emphatic about the peril of going it alone, then if one of two cofounders leaves, the remaining one won't want to continue.
