Wikipedia says that the amount of lead in 100LL avgas is "about 4 times what was in pre-1975 leaded automotive grade gasoline."
As a private pilot, yes I do feel a bit guilty that my hobby adds lead to the environment ...
A big part of the problem is regulation and the certification process. Of course there is a good case for heavy regulation in aviation, but particularly in low-volume general aviation costs have been kept too high for too long that it really has stifled innovation. We are only beginning to see modern engines in light piston aircraft.
There's a lot of resistance to changing anything around aviation, especially when it comes to engines. Many (or most?) piston-powered aircraft are single-engined aircraft, where a single failure makes the plane a glider.
Getting power out of a certain quantity of fuel is also a big plus since the 100 liters of leaded avgas you carry can take you 600km but unleaded maybe only 500km.
Frankly I don't think piston-powered GA is vital enough to our economy/society to justify such a long grace period on removing the leaded fuel.
If our Cessnas can't climb quite as fast or fly quite as far - that seems a very reasonable cost for removing a known potent neurotoxin from the environment.
I just hate to think of all the avgas I got on my hands when checking the fuel before each flight, dip the wood stick in the top tanks to check teh level. But even worse is when you use a small graduated cylinder with a metal spike to drain off a good 100ml of fuel from the under-tank to check for water then throw it on the ground! The taxi area must be loaded with lead.