I mean, at this point Kagi could probably start their own, and there are plenty of smaller indexes (Mojeek, Yep, and Marginalia come to mind)
I know this wouldn't be easy but maybe such an arrangement would be possible where there is some shared index and then the partners can each use it, extend it... in their own way.
I've often switched between Google and DDG but always came back to Google as in direct comparisons I was always not finding the right results in DDG. Kagi is the first time that I have not switched back a single time as nothing changed negatively in the quality and quantity of the results.
It's a noble goal to have your own search index, but it's duplicating a lot of work that others with much more resources already do well.