A common argument is that it's valuable to have a partner as a sounding board and as a source of moral support. These roles are vital, but they don't necessarily have to be filled by founders. Einstein needed the help of his friend Michele Besso to develop special relativity, but Besso wasn't a coauthor on Zur Elektrodynamik bewegter Körper.

I don't see two as a natural lower bound on the number of founders. As the productivity of individual programmers continues to rise, I expect to see an increasing number of single-founder startups succeed.

I have also come to the same conclusion independently.

You need a sounding board and a source of moral support absolutely. This is best done by a co-founder. However, it CAN also be done by someone who is not a co-founder and more of an advisor.

Also, you do NOT absolutely need co-founders with complimentary skills. That is a myth. Larry and Segie did not have complimentary skills. Bill Gates and Paul Allen did NOT have ALL the skills required to build their business. They had or developed (read learned) a lot of skills on the way, for which they could not hire immidiately - which is the only thing that is required of a founding team.