You make it sound like there's a tonne of FOSS community on-hand just waiting for the chance to step in and create a popular fork that will lead to fragmentation, when he's effectively been the sole maintainer since 2012 [1].
It's far more likely there will no future well maintained OSS forks, he'll continue developing it as a closed-source product, allowing end-users to install it for free but any company who wants access to his future source code and improvements can compensate him for it. This is a far better outcome than it turning it into abandonware where that will stop working in future macOS versions.
> software won't be available for public scrutiny.
It's a popular cliche to think all OSS projects have healthy development communities and many eyes pouring over it, but as he's the only person contributing fixes, the only way the product was going to improve is if he spends more time working on it which is more likely to happen if he can get sponsored to continue working on it.
None of your fears are likely to come to pass and the health of the project would be far worse off if he abandoned it.
[1] https://github.com/osxfuse/osxfuse/graphs/contributors