That is small change compared to the council borrowings required for basically a new airport connected to the existing runway.
Much of Luton’s growth has come from Eastern Europe so they would have to find new markets to keep on growing. With its 2160m runway Luton would be excluded from many airlines potential long haul plans and Luton’s traditional bottom end market would not suit most national flag carriers running overflow services out of Luton.
Think we could expect another low rent terminal with a split easyjet Wizz operation but I am not sure where other growth would come when Stansted will offer a better product using its longer runway.