Dop, I live in Sheffield, and as far as I know, they intend to keep the airport suitable for GA, though with a runway length of 640m. It will take a couple of years to reduce the current length to that as development progresses.
Unless you intend to fly a jet, I wouldn't have thought 640m would be a problem?
One possible worry for the long term would be that they intend the new development to be part-residential - no doubt those new residents will raise noise objections as soon as they move in.
- Michael