That modest loss of power may not be acceptable for a twin engine plane due to loss of any single engine climb performance. A lot of the older airframes that are still flying in Pt135 and pt 91 are there because the manufacturers have not produced a replacement for the jobs those airplanes do, or the ones that have been are too expensive to buy or to make a profit with. I do agree with the proposal to stop certifying new airplanes running on 100LL. It would be a good place to start with the final phaseout of leaded fuels.