Serious question; do those engines that can now use this already have hardened valve seats?
They are standard aircraft engines - the hardened valve seat thing for older cars and motorcycles using unleaded fuel was largely a UK theory, promulgated after some areas worldwide had been successfully (and by necessity) using unleaded in unmodified older vehicles for years.
I've had no issues with valve seats in using unleaded fuel to fuel many of my vehicles for about 30 years, including about seven currently in hand that were originally designed for leaded fuel. Two of them are unmodified Continental and Lycoming aircraft engines that were STC'd for unleaded auto fuel about 25 years ago, and would be fine with it now except for alcohol now mandatory in my market.
I think 100UL will transition into the market when the oil companies figure out how and when they'd like to do it, and replace 100LL. Relatively complex, expensive Diesels will maintain their appeal in those areas where GA largely doesn't exist. But where GA is more widespread it makes more technical sense to use a more refined, higher performance fuel.