On one hand single-founder startups don't get as big, which is a bad thing. On the other, writing code for 8 years as a single founder is right about the definition of doing things that don't scale, which is a good thing. So you can't say Colin running Tarsnap as a single founder for 8 years was bad for Tarsnap.
But it also sounds like unexplored terrain. The more specialized a skill you need for some types of work, the more likely this scenario is to happen. What's missing here is a way forward. If the goal of Tarsnap is to grow, the north star is Tarsnap's growth rate.
Colin, could you share more about what you are looking for?