However, everything is subject to change, but I think past the moonrise towers it will be hard to return to the earlier areas.
This is a fairly standard Larian design feature going back forever. That's not to say it won't change (and personally I hope it does; I like being able to go back to where I've been, even if "just because") but I would anticipate that's how it'll be.
Is it really? Out of the few Larian games I've played, this was only a thing in DOS:2. You were allowed to go back to previous areas in D:OS1, and you also could in Divinity 2, although the earlier areas in the latter game became completely altered and couldn't be crossed on foot later on.
Whether it happens in BG3 or not is probably going to be a byproduct of the story progression and setting. DOS2's plot was anything but self contained, with transportation between all of the acts being done by boat. Right now, BG3 seems up in the air in comparison. Yet the idea that there are three distinct paths to get to Moonrise Tower (that are very likely to be mutually exclusive, I imagine progressing too far in one will cause certain events in the other paths to progress to a point where you're locked out of them), and that the only accessible path thus far forces us to take a boat in a very heavily implied 'point of no return' kind of way seems to suggest that we're most likely following a DOS2-style structure.
This isn't really a bad thing, so long as there's an actually good reason for it. Pathfinder WotR for instance has a lot of points of no return (more like you can't re-visit the end of chapter dungeons).
