The motivations and reward structure for the miners are extremely relevant.  The Nash equilibrium is about distribution of resources, and is not affected by an increase in the supply of a resource that is currently plentiful enough that it hasn't materially affected the dynamics of equilibrium yet.  

This proposed fork would not disturb their present equilibrium, but actually being limited by the block size limit when the time comes that they want to create larger blocks definitely would.  If they find a new equilibrium in which the limitation on block size plays a role, then the proposed fork would disturb that new equilibrium.