That's where you're wrong. Most open source projects have a single maintainer who does the vast majority of the work. It is not uncommon for them to put in 30 hours per week or more for free. In theory someone could step up and replace them - but no one ever does because that requires actual work. Maintaining a large project is a thankless task. People are constantly bitching and complaining why you haven't implemented useless feature X or fixed random bug Y for their commercial product. Github is full of self-entitled whining users who never give back to the community and expect everything for free and fixed in a timely manner.
Large open source projects being driven by unpaid volunteers is simply not viable or sustainable. All big projects (Linux, GCC, LLVM) are sponsored by the likes of IBM, Google, Facebook, Red Hat and Apple. It's the only model that works.
There are viable large volunteer projects. The way they become viable is to interpret single points of failure as damage, and to fix that damage.
That damage definitely includes relying on a single maintainer's ability and knowledge to keep the project going. There's even a term for it-- "bus factor"-- which answers the question of how many of the core developers could get hit by a bus with the project remaining healthy and maintained. If the "bus factor" is 1 and the userbase is large, that's a critical bug in maintainership.
It's not like he's irreplaceable, even if there's not currently a successor.