Car engines routinely have compression ratios (CR) of 10:1 or higher and run fine on mogas. In significant part, it’s because of far-better engine management systems (EMS) which are totally lacking on traditional aircraft engines. But fundamentally, it’s possible only because their small cylinder volume automatically makes pre-ignition much less of a concern. Upgrading from 1928 ignition systems to EMS is technically quite easy and relatively inexpensive, but FAA certification is a byzantine and expensive proposition. Reducing CR with shorter connecting rods is also relatively easy. Turbo engines already have mechanically-lower CR and could be “fixed” by limiting boost. Reducing the effective CR reduces power of course. Expensive true.
Other regulations presently make adoption of 94UL difficult. A new standard self-fuel installation is very expensive and fuel volume isn’t enough to easily make up costs. (The few in my area charge more for 94UL than 100LL as a consequence.) Numerous and varying local and state restrictions generally forbid having small private fueling options. And, as is the case with me, even with a lead-free loving Rotax, I am required to burn 100LL because that’s what the USA certification certificate requires and the manufacturer has nothing to gain now by changing it, nor is there enough money to be made by going through an STC process, for a tiny number of such aircraft types.
Overall, a pretty bleak picture.