Swift UL94 has been to market for 3 years with very low uptake.

Fair enough, but that's not a replacement for 100LL since not all engines can use it. It's gotta be a pretty large FBO to have enough market to pay for setting up a distribution for UL94 in addition to 100 octane gas. Like, there are engines that were certificated to run on 80/87 avgas but no one sells that any more either. Today's avgas market just isn't large enough to make it worth offering two fuels.

About 2/3rds of GA has an STC available to burn it.

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.

Because as far as I know the engines must have their type acceptance updates to reflect it's allowed use.

3 years is also basically a second at the speeds GA changes at.

Yes, technically speaking.

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.

The slow uptake has more to do with it just not being all that available.

Yup-- why should an airfield bother with going to 94UL when it's more of a hassle (some planes can't take it, all planes need an STC).

But if you taxed the crap out of lead-containing fuels, I suspect more would be interested...