Unfortunately, using a BGS and p2p instancing is the only feasible way to have something close to a single universe game. Current computing and networking capabilities of the average consumer does not permit anything else. The only possible option is to run the game on a server cluster and stream to clients but we have a ways to go for it to become an affordable option both for developers and players. At that point though, monthly subscription fees will be inevitable.
On the bright side though, with a little bit optimization and implementation of some novel ideas on the instancing and NPC persistence, I believe we'll have a much better experience in the following years. We don't need too much else on top of what we have now if you think about it. Just better consistency with getting matched to people we want to play with and more organic way of npc spawning.