I think it sounds good only until put in practice.
If 99% is useful but 1% goes against the instance's philosophy, they have to accept this 1%. But if someone involved in picking what is put in this overarching moderation starts pushing this 1% to 2%, 4%, ever more through the Overton Window, it would deteriorate more and more the instances that had to accept the original 1%.
And even without the possibility of intentional sabotage, one rule being applied for every individual instance would still mean instances would need to adapt for the lowest common denominator, meaning a slow end of individuality. And if a small instance is no different than a big one, those bigger ones start canibalizing the userbase of the smaller ones.
What I expect to happen, and that already does to some extent, is to clusters of instances with similar interests to form, and by extension being prone to accept aligning moderation and philosophies with their direct allies.
But that's still mostly a bilateral move, not an one rule to govern them all, so dependence or independence is still tied to individual interests.