I think the airport will start to reach a sweet spot from around Christmas 2017 as most of the fixes will be in place. The CTA will be finished together with a new tarmacked short term car park, new multistory car park and road network. The terminal extension will be finished and hopefully the new pier plus another dozen or so of upgrades and infills. There will also be some slack at the airport as it will be below capacity.
I have no doubt that Luton will hit 18 million passengers but as that figure gets closer the airport will start to creak again. Also, as what is happening now, the airport won't be turning anyone away if that figure is busted.
Hopefully plans are being drawn up now for expansion beyond the CTA because I can't see what more could be done to expand a fixed space.