But it is also incorrect to draw a parallel to cars, because there was enough volume for cars to transition gently to unleaded gas; you could get both for a while. There is not enough volume of 100LL for FBOs to have two tanks, so, the switchover will have to be all at once. That's why the "drop-in replacement fuel" search is going on... Don't know if it will come to fruition, but if it does, sure makes the problem a lot easier.
Today, though, it's frustrating to have an airplane that I know could run fine on ethanol-free, lead-free 92 octane motor gas, but am required by law (unless I buy an STC) and by practicality (what airports sell) to use leaded fuel.
I guess I'm assuming that an engine that's been converted for unleaded will still work on 100LL. Obviously that's the case for yours, but I suppose it may not be the case for all. If so, that would complicate things severely.
Most normally aspirated engines will work fine (in some cases, better) on a high grade unleaded auto gas.
Most turbo-charged engines lack sufficient detonation margin when run with that "premium" auto fuel, and need some form of octane boost to provide adequate detonation margins for a high-temp, max performance takeoff run. Several groups, including Swift, GAMI, and Shell, are working on drop-in zero-lead replacements for 100LL. All are flying the fuel in experimental applications as I understand it. None appear "close" to certification.
The real problem is that the bulk of gallons burned are burned by airplanes that are turbo-charged. So, even if 75% of the engines can be readily converted (in many cases, a paperwork-only STC), those 75% of airplanes likely represent fewer than 25% of the gallons burned.
I've been to GAMI in Ada, OK and seen them run their no-lead fuel in a lab setup. It's impressive and looks like it would work. They still face a steep and long uphill battle with the FAA, as do Swift, Shell and the others...
I'm sure it won't be cheap. And this isn't just an abstract cost to me; although it won't hit me nearly as hard as some, I do belong to a flying club which owns two airplanes that would need the work. On the other hand, owning an airplane is painfully expensive anyway. Perhaps I'm off base, but it sounds like the unleaded retrofit wouldn't be too painfully expensive when, say, combined with a 2000-hour overhaul.
I'm sure there will be a number of aircraft owners who couldn't afford it and would have to sell their plane at a discount once the fuel cut over. I'd feel bad for them. On the other hand, they (we) are running pretty dirty machines that emit some nasty pollution, and it shouldn't be a big surprise that they (we) can't get away with that forever.
On one hand, the small size of the GA fleet means that this is a tiny problem compared to leaded gas use in cars. On the other hand, the general reduction in pollution in the US means that it's substantial relative to what's left. Leaded fuel used by GA accounts for almost half of all lead emissions in the country now, and that seems worth fixing.