Data seems to imply that the diesel engine lifecycle cost is slightly better now than conventional engines, so electing for a diesel replacement instead of a conventional engine overhaul can make sense... but it's a razor thin difference.
If you made leaded fuels more expensive-- more airfields are going to want to move to UL94, and more people at overhaul time are going to go the diesel path, and the problem will gradually get better.
There's not going to be one single tidy drop-in solution for the entire market. That's why we should tax leaded fuels, and everyone can pick the appropriate solution for them. Some will pick diesel conversions. Some airfields will go 94UL and people will purchase STCs. Presumably some other people will take other paths. And those with the hardest time transitioning can instead pay the tax and burn leaded fuel, still.
3 years is also basically a second at the speeds GA changes at.
Swift charges $100 for a STC, and has coverage of two thirds of the GA fleet.
Really, all that needs to happen is an airport needs to decide that lead free is important to them and they'll pay the premium for Swift's fuel.
But if you taxed the crap out of lead-containing fuels, I suspect more would be interested...
There's an increasing number of diesel engines for GA aircraft, and supplemental type certificates to retrofit them into the same. If 100LL got more expensive, you'd see more people opting for the diesels when an engine reaches the end of its life.
e.g. http://www.continentaldiesel.com/typo3/fileadmin/_centurion/...
Yes, it'd be really cool to end up with a lower-lead fuel that's safely combined with 100LL. That's been "imminent" for the last 25 years.
The poster above advocates for a "rip off the band-aid" approach. GA advocates for a gradual, painless transition that, in practice, will never happen. Surely there's some middle ground?