I do believe they underestimated a little the networking performance and hardware needed to sustain so many instances within 400 billion star systems. I mean even if an instance isn't there the game still needs to be throwing so much data around.... players, NPC's, BGS and trade data.
ot but this has nothing to do with the number of star systems. also, bgs and trade data (and even star data) are not relevant here since they are relatively static information. the realtime update of rapidly state changing elements (players and maybe npc depending on how deterministic they are) is the bulk of the network traffic and, since it is p2p, it increases geometrically with every additional client in the instance.
the trick you mention could indeed work if it causes peers to handshake through the same server (?) so they have a better chance to instance together, but that's not what op is trying to do, which is: once in the same instance getting the traffic between those peers to route locally since all are connected to the same router, and not through the internet.