In practice, you’d need some redundancy because the admins will also need time off, vacation, get sick.
So, I am not disputing that 8 FTE is too much. What I want to make clear is this: there is not a single instance out there that is getting enough money in donations to pay even one admin, which is a clear indication that the model is not sustainable.
Your question is as short-sighted as “If global warming is real, then why is it snowing in Southern Europe?”
No, a system that is not sustainable does not imply that all the ecosystem dies simultaneously. It just means that it relies on a continuous stream of idealistic people coming in, willing to help, only to collapse eventually later.