Surely there's some sort of caveat to this. I am very skeptical of such a service. If I need to keep a domain renewed I pay for a 'rollover renewal' for 10 years. When that 10 years expires, I just renew for another 10 years.

There is a big caveat. A registrar can not register a domain for more than 10 years. There are no exceptions to this. That means we are trusting the registrar will not go out of business and trust that they will always be able to renew every 10 years on our behalf. There are a few registrars that do this. I consider it to be a form of speculation.