When they looked at 10/28 they didn't look at new markets they looked at the markets they currently served and tried to anticipate the future level of demand using the same aircraft type.
Unfortunately they only looked at 10/28 in terms of capacity. They found it was similar to Gatwick and Stansted but regrettably they didn't even consider aircraft type or requirements regarding length to open up new markets.
They nearly got a new parallel runway out of it so how could extending 10/28 have ever really been on the cards?