I think the single map design is beneficial for multiplayer. In my every coop session and freedom it offers does become helpful - in the very least, that for example a coop partner can hop into town and do some shopping without me having to tag along. It is also a unique feature and design of Larian so I doubt it will go away.
That said! If they stick to it, they could design areas with that structure in mind. Act2&3 of BG3 felt far more natural, then act1. A big city consisting of few large maps with stuff tightly packed together, feels like a fair representation of the setting - a countryside where to factions can't locate each other less so. Perhaps also changing perspective when exploring could also make those smaller areas feel a bit more sprawling.
