There is no minimum safe level of environmental lead. Even small amounts can have devastating impacts on children. More and more environmental activists are documenting residual lead near GA airports.
Banning lead from fuel is a no brainer for politicians as there is no down side from their POV. I am frankly surprised that leaded Avgas has survived this long and so now the challenge is going to be an orderly transition to a UL 100 without local supply outages during the change over.
The thing I am afraid of is that GAMI has a monopoly on UL Avgas. The overall market is probably not big enough to encourage serious competition and so I think keeping Avgas prices reasonable is going to be a real issue.