My swag: About 60-70% could hold a medical to 67. Assuming they wanted to, vice going out or staying out on LTD.

Maybe half of those would care to do so. A lot less than half if you have to downgrade from international WB flying to stay, which is the case with current ICAO rules. That rule might change quickly if the US went to 67, since we probably account for close to half of the world's airline pilots.