That is indeed a problem which has yet to be solved. Or a potential problem, rather... I'm rather hoping it will never actually happen. ;-)
Seriously though, it is on my list of issues which needs to be addressed. Bringing in someone else and getting them up to speed on how to run everything is an expensive prospect, though.
Have you considered some sort of "enterprise" variant where large organizations use their own storage backend? Just 1-4 serious enterprise sized customers would cover the salary and overhead of 1+ good engineers. There's a lot of opportunity in that direction. Like any medium sized or larger company that needs to deal with compliance with education or medical privacy regulation, your tech is a great backup solution, and if carefully done doesn't increase your overheads much/ at all.
Setting up tarsnap to use non-AWS infrastructure would be a significant amount of work. Setting up a "private" Tarsnap (but still on AWS) is something I could do for a company needing to store a large amount of data (say, 10+ TB).
doesn't AWS have some special clouds for companies that have compliance needs?
Either way, it might be worth looking into even just the "private" Tarsnap on AWS business direction as a way of growing revenue in a way that isn't tied strictly to data storage volume.
One way to go about this is to ask some of your larger business users if they would be interested in such a "private for them" self hosted Tarsnap variant. I think many of them would love a way to help you have revenues sufficient to support having an additional engineer (or two) working with you, which isn't possible for them to do with your current usage based revenue model.
Point being, theres probably an "enterprise" business model that stays true to your quality goals, but gives you more ahead of time revenue by a substantial amount. For some of your customers, there might be more value in supporting you being able to hire some engineers than there is in the cost savings element of the current revenue model. This can be an ancillary product that isn't the core one, but which still helps you have more resources to make the core better.
Talk with your larger customers, they're probably happy to chat with you given the chance.
Seriously though, it is on my list of issues which needs to be addressed. Bringing in someone else and getting them up to speed on how to run everything is an expensive prospect, though.