He's definitely assuming that this commercial version remains primary:

"I will be forced to continually improve it and upgrade if I want to keep a for sale version without there being an equivalent FOSS version around (which in theory could be maintained by the community)"

But, is mostly discounting a community fork, or divergence between his commercial version and the older GPL version. If he receives patches he'll have to have a copyright assignment policy so that he can apply the features to both trees - which will risk a fork as many people strongly dislike them.