I think the main reason for decrease is GA pilots is due to the high costs. I have wanted to fly all of my life but only once I reached my 30's could I even consider it. I'm also sure a lot of people who start their PPL end up not finishing because the cost of flying plus all the hidden costs quickly start to spiral out of control. Now that I have been through the experience, it became apparent that when schools quote a price to get your PPL, really you should add another 30-40% on top of that because of all the extra hidden costs. Even things like landing fees, airfields charging £15 to £30 for a single landing just seems crazy. Another possible factor as well (at least for the UK) flying in the South of England can be quite stressful at times because of all the airspace, restricted areas, temporary airspace, danger area etc etc. Sometimes it becomes more of a chore than a fun flight because you have to plan your flight so carefully and sometimes at the last minute something comes up in the NOTAMs causing you to have to change your plans. On the cost front, there may be a glimmer of hope. Look at all the development being put into electric aircraft. Take the Pipistrel electric for example. It's a 2 seat trainer which supposedly costs 2 euro per hour in electricity to run. I'm sure that will also make all the eco-warriors (trying to shut down all the airfields) more happy if the planes are zero emission and a lot quieter.