Cambridge played a huge roll as operational airport for Ryanair
and GO/Easyjet when Stansted played host to the hijacked afgan
aircraft, flights Arr/Dep Cambridge with full loads so the runway is not a problem as far as I can see, however the flights were all checked in and boarded onto busses at Stansted. Arrivals were fun with the one tiny 3-4 foot long baggage reclaim belt and they only had one set of a/c steps
for the B737.
If and its a huge if, FR or EZY were to move some/all operations from Stansted to cambridge this would be a very long way off and would also free up some much needed slots at Stansted, I cant see any airline giving up these prime slot at a major London airport with good road and rail links to the city.