They're working on replacing leaded fuel. The challenge is mostly regulatory, since they won't certify any of the replacements until they're sure it's 100% safe. It's still a work in progress.
As for what you'd lose: Search and rescue, medical transport (both commercial and volunteer), transport of blood samples for lab testing, high resolution aerial photography on your smartphone, powerline inspection to prevent forest fires, crop dusting. I could go on...
Same is true of Medical Transport except possibly for Australia where light aircraft are used more frequently than turbine helicopters.
Same for powerline inspection, although there could be a couple of companies using Robinsons somewhere I guess.
The problem comes with flying in lower atmospheric pressure (high or hot environment) because the replacement chemical used in place of the lead-based additives makes fuel much quicker to evaporate.
BTW, the really rich don't use AVGAS 100LL except maybe for hobby flying - kerosene-based JET A-1 is used in all turbine engines, as well as the rare diesel.