General aviation has been on the decline primarily because of the way US liability law is structured and the cost of FAA airworthiness certification. A quirk of legislation made it such that old models of planes are shielded from lawsuits, among other issues. If you are a manufacturer, why bother designing a new plane when you can keep selling the same world war 2 era stock? It's not like you will have any competition. The cost of certification is also why Boeing tried to avoid certifying the 737 max as a new plane.
Fewer people are learning to fly except to go work for the airlines. Most of the small aviation manufacturers and engine producers have been snapped up by China. It's so expensive to certify an engine for flight that most of general aviation is using engines where you burn leaded fuel and manually control the carburetors and fuel injectors. In aviation, a 1940s era airframe is considered perfectly safe, and it is completely acceptable to use vacuum pneumatic instruments despite them being significantly less reliable than MEMS or solid state ring laser (or fibre optic) gyros. Even something as simple as a radar altimeter would cost well into the 5-6 figures. It's a very backwards industry similar to medicine and pharmacy. The doctors believe in hazing new grads through residency, while biotech legislators holds the view that right to try and loosening clinical trials lead to moral hazard; better ten people die from disease than for one to be accidentally killed during drug testing. It is the same in aviation, safety is written in blood and all that so the best way to stay safe is to make it too expensive to fly. Many flight schools preach training with old airframes and technology. A lot of pilots refuse to admit the real reason is because of cost, and instead come up with all sort of post-hoc fantastic explanations that students learn better when they are flying rickety vehicles with century old technology. If you want to learn to fly, find a school that offers the DA-20/DA-40 (or even better, the DA-40NG with push button start) manufactured in the past ten years. Avoid the Cessna-only places.
The FAA is trying to improved the situation with the new MOSAIC light sports aircraft policy (that's mostly half baked) and certifying more engines for unleaded fuels. But unless the White House comes down hard on them for their inaction, they are going to drag out the issue for another decade (and hopefully kill off the general aviation industry all together). Regulators don't like general aviation, especially post 911. Between drones and climate change, and the fact that general aviation pilots are mostly Midwestern farmers and upper middle class people curious about aerospace, the industry doesn't have any real advocates. Not enough nouveau riche tech bros fly, and the ones that do are often rich enough to buy their own legislators and presidents that the costs don't matter. It's the aviation version of the middle class being squeezed from both ends.
If general aviation continues to decline, it's going to become a national security issue. It's not a good world where aviation becomes a professional luxury, where aviation manufacturers cater primarily to foreign clients, and where most aerospace engineers would never have the opportunity to actually fly what they build.
