But until that point, you might have a good enough product: One that has momentum and requires effort to corrupt, that users are aware of and have expectations about, and that presents value that people otherwise might not have known is possible. Control being lost doesn't necessitate that all the value & impact is lost with it.
He might not, but 6 months later the company he sells to *might. Or if public could be taken over with little choice in the matter.
As you say, control is gone once he sells.