Flyf15 pointed out some of the reasons, I'll elaborate on more.
The biggest is cost. Certification of aircraft and systems that go in them is astronomical- air conditioning is a $20,000 option on new skyhawks. That is not a misprint, 20 grand. Why? Because Cessna had to prove that it wouldn't adversly effect performance in any failure mode; it couldn't get stuck on, in any way. And there is a very small market to recoup this investment on.
GPS is getting very common in GA aircraft; most new airplanes come with it standard, and its getting harder and harder to find a privatly owned plane without any kind of GPS in it.
Some of your other ideas though, just don't make sense in all airplanes. Example- spoilers. SOME Mooney's have them because they need them. They are very clean designs that are very notorious to slow down for descent, and they have a low gear operating speed. Most GA singles just don't need that kind of extra drag on top of flaps and gear to slow down.
Another reason is safety. I know I wouldn't fly a GA airplane that had some kind of automatic lift dumping system installed that was activated by a microswitch on the gear; too many bad things can happen when it activates at the wrong time. And I can garuntee you that that kind of retrofit would not be cheap, retrofit on a Mooney is close to six grand.
And then, why would it be needed in the first place? In most light aircraft, speed brakes are used to slow the plane down and descend without having to pull the power back much. They aren't used like on airliners to dump lift. And bounces are trained-away for the most part. No need for expensive solutions to problems that aren't there; I don't think I've ever heard of a fatal bounce, and I've seen/had some bad ones.
Piston engines are going to be around for a long time for light aircraft though. Turboprops just aren't affordable yet in the 90-300hp range yet. There are conversions, but mainly for planes that are designed for traveling long distance. The turbine engine shines when it can get up to the thin air where it can take advantage over its greater power to size ratio to make up for its attrocious fuel burn. I don't think you'll ever find a turbine powered 172.
Safety has made large leaps forward in the past, but most improvements will continue to come in the form of pilot training. Perfect example is the new Cirrus airplanes; they have the "foolproof" airplane saving parachutes installed, they've been involved in 5 accidents since they've been introduced, only one has used the parachute successfully.
The problem you run into with trying to make a light GA plane "pilot-proof" is that many people who purchess and operate them do that because they WANT to be pilots. We like knowing that the sucessful outcome of the flight depends more on our skill and training than the fancy gadgets designed to make the airplane foolproof.
And you want bad panel layout? I've flown a modified J-3 Cub with bush tanks; long range tanks installed in the wings (the orginal tank was located between the firewall and the insturment panel, sitting in the front seater's lap.) To reach the selector valve from the back seat involved unfastening my harness and half stand/lean over the front seat. The solution is to not run a tank dry at 200', and to be aware of this limitation of the aircraft.
Light aircraft technology will continue to be in terms of evolution, not revolution for a number of years to come.
Happy contrails - I support B747Skipper and Jetguy