I agree. We had to switch to fine-wire plugs in one of our club planes because for whatever reason it was particularly prone to lead fouling (neither of the other two planes were). The plugs we’re using also have a shorter lifespan, so I’d estimate over a year or maybe two, we would have already broken even–if not gotten ahead–on the cost of the STC. And that’s not even taking into account longer oil-change intervals that would be possible with an unleaded fuel.
As soon as there is availability of G100UL in my local area (“local” meaning, within ~150-200nm), I expect my club will purchase the STC for all three planes.