I'm not too concerned about an infinite regress - Lewis Carroll has an answer to that in a paper from around 130 years ago, and in my view that answer can be applied outside of the formal logical domain Carroll is concerned with to other domains in which rules operate.I don't see how deconstructing the philosophy of rules will lead to anything more than a nearly infinite regression (which I see as a digression). But if you and @AbdulAlhazred and anyone else (possibly @pemerton given his position as an academic philosopher) find it useful and/or at least possibly capable of stimulating the penetration (giggety) of Gamist-related design and experience of play?
But I do agree with you that it is a digression. The Lumpley Principle - ie that it is social contract that establishes the system, and that it is shared imagination that constitutes the setting, characters and situation - is (in my view) uncontroversial, and I haven't noticed anyone in this thread trying to controvert it.
But the Lumpley Principle doesn't tell us anything about what systems are possible and the differences between them. Baker himself - as per my quote upthread - clearly thinks that social contract can support a system of task resolution, in which the resolution of situations is under the control of the GM who (as @Campbell explained with reference to John Harper) must decide what affect (if any) the achieving of a task by a character has on the unfolding and ultimate upshot of the situation; or a system of conflict resolution, in which the resolution of situations is not under the control of the GM in the same way.
Baker (and I think @Campbell too) have mostly made the point that that task-resolution, "GM is the glue" approach is not very suitable for "story now". You (that is, @Manbearcat) have also made the point that such a system is not very suitable for "step on up".
We can posit exceptions. A very disciplined GM, with very transparent fiction (setting, situation, NPCs), might make task resolution work for vanilla narrativism. I don't think it will be trivial. Rolemaster looks superficially like task resolution, but some of the features of its non-combat resolution charts actually push it closer to conflict resolution in some arenas (especially social). Nevertheless, using it for vanilla narrativism still raises some of these GM-as-glue problems. I say that on the basis of nearly 20 years of experience with the system.
On the gamist side, a commitment to rock-solid prep and a tight resolution space (ie the dungeon) can make GM-as-glue gamism possible. Even here, there can be problems, for instance for some social conflicts. And take the same approach into a less confined and spartan (imagined) environment, and as I and @AbdulAlhazred have said, the gamism will break down, because no matter how disciplined the GM is, they will have to make stuff up to preserve the logic and verisimilitude of the fiction.
I therefore don't think the rather modest exceptions refute the general point.