This seems like a problem of the search space rather than solely Google's. How are you supposed to guard a system which, by its nature, provides financial incentive to others to game it?

No matter what weights and checks are put in place, some observers will notice how their rankings change and make appropriate modifications.

This isn't solved by Kagi's product, either. I'm a happy user and use it as my daily driver. That doesn't mean that, if it increases in popularity, the results will remain unskewed.