Manston was also used as a training site for the airlines as well as flight training by TGA so there could be a few revenue streams coming into play once it's actually open. It's ideally situated for a training organization to offer both UK & EASA licensing if there continues to be a need for that.
I'd imagine Riveroak have been in talks with cargo operators and airlines to gauge interest and deemed it enough to go ahead. In which case this would far outweigh any report done years ago.
Listening to the most outspoken opponents of this project there appears to be 2 main arguments,
1. That there will be 747 freighters every 15 minutes screaming overhead Ramsgate causing noise and air pollution.
2. That there is no possible chance of success for a reopened Manston and there will be no freight or pax requirement to make it viable.
These 2 arguments cancel each other out since if there is no flights then there is no pollution.