As someone who has used EDI in excess of 150 times and at least once a year since about 1996 and as someone who has worked there I agree it's growth rate has been exceptional and it has simply failed to keep up. There has never been a desire to actually extend the terminal in any real, meaningful way and the problems that needed addressed, mainly the check in hall and arrivals process have been utterly swept aside. They have used gimmicky things like self bag drop and podium style desks...which has created more room for more airlines...but...there are now more people crammed into a hall designed in the 1970s.
I do think the departures process is nicer than the likes of LTN for example, which is just a horrible airport all round really. LCY is not that great either. At least EDI does have quiet spaces to go to and sit. In the morning rush, perhaps it's limited, but come about 11am the areas on the SE Pier and around the 1 gates become pretty quiet for long periods of time between flights. There is nothing like that at LTN with its centralised cattle pen and almost no seating available anywhere else, or LCY where the lines for some shops and food outlets are so long I'm never sure where one ends and new one begins.
That said, the major problems with EDI sourround the arrivals process. Domestic passengers still need to traipse through the gate area and mix with departing passengers. Not a security risk, but a major cause of congestion, in effect there are often nearly double the amount of passengers airside than there needs to be. The international arrivals process is the shame of Scotland, for sure. I am utterly ashamed to think this is what welcomes millions to Scotland each year, especially the cow shed that is International arrivals 2, which doesn't even have a real roof, floor or a single sign showing you where to exit. The walk for arriving passengers on Stands 10A, 12 and 14 is a mess, actually quite unbelievable this was even made to be just so amateur and unwelcoming...it's not even weather proof...one 12 and 14 not even covered!! This is where some of EDIs biggest stands are, so are used by their most prestigious airlines, namely QR and EY, but often also AC and AA. This is also where any future long haul flights would be serviced, an absolute joke. Having said that, the original international arrivals experience is not a lot better. With that corridor set up, which was not a good idea in the 1970s and certainly isn't a good idea now, leads to congestion, delays and misery.
They simply needed to build a new terminal about 10 years ago, but it's too late now. All they can do is build on piece, by piece and slowly change the layout of the airport. The SE Pier was possibly the biggest wasted opportunity in the history of EDI. Had that been built with a new check in hall, new contact gates and much better infrastructure in general, they would have had the space to play with to sort out the older parts of the terminal. The baggage system needs a complete modernisation too. They are still using paper bingo cards and the actual baggage belts break down so often, for both arrivals and departures. I just don't know where they can start.