Stansted to Central London in 25 minutes would certainly attract a huge range of new airlines
Gatwick to Central London is struggling to deliver what you see happening at STN and it's had that same rail link decades. It's not transport links that are the problem at STN, it more to do with the fact it's the airport the market never asked for, won't use and still shuns. The Ryanair operation exists as a last throw of the dice from BAA after every attempt to get airlines to move from LHR failed.