> One issue I see here is that time protection would need to extend to anything shared,

We might see this issue as an opportunity. That is, by thinking about a concept called "time protection" we expose all these things subsystems are doing and make them easy to argue about. We can now say "Oh good, XYZ improves best-case speed, but sadly it also compromises time protection".

Having such a language means the industry can slowly start improving these things rather than sweeping them under the rug. It will not stop the improvement from being slow and difficult.