I mean, GM mode is specifically for on-the-fly "I sure didn't expect that" things the players do. I don't really see it being the end-all be-all for worlds. It's not really designed to be something like NWN2 stillh as to this day with persistent multi-map 40+ pplayer worlds that can ALSO have GMs for non-scripted stuff. It's pretty explicitly just having a GM to handhold the players through every interaction.
Whereas creating full campaigns with scripted NPC action and such is just... y'know... for scripted, massive worlds with their own set paths and yadda yadda blah blah.
The GM mode is a social thing for a small group of friends to have a pseudo-tabletop experience. Nothing else, not even from advertisements.
I fail to see why Larian shouldn't go the extra step to allow scripting. For all we know, it took extra effort to actually disable scripting in GM mode. A vision of "having a GM handhold players through every action" and "a social thing for a small group of friends to have a pseudo-tabletop experience," is a pretty unambitious vision if you ask me, and Larian is far from unambitious.
I don't expect persistent worlds with 40 players, I just expect the mode to support the basic scripting functions that the default game already enjoys. And if they go for basic stuff like same area transitions, I don't see why they shouldn't just allow all scripting. All the infrastructure for scripting is there. Give GMs all the tools they could need, instead of arbitrarily limiting them.
Unless I hear a very good technical reason this isn't possible or very difficult, or a well-justified argument for disabling scripting altogether, I'm going to keep harping on this.
