extending the runway etc is a damn site cheaper than rebuilding the terminal - and both are needed
The terminal building itself is big enough, the problem is the use of space within it. On the first floor of the building sit all of the management offices. Throw those out to somewhere off-site and then you relocate the security search point and executive lounges upstairs, freeing up lots of downstairs space to extend the retail offer and departure lounge. Stands will be a constraint, but there is scope to add some more up towards the cargo area and to extend the small aircraft pier up to Bristows to add another two or three pier-served stands for the IAC flights.
Any building project at an airport is expensive, but as projects go, this airport has a lot it can do to make best of use its current facilities before needing to splash the cash on massive building projects.