I believe they decided to do this not bcs of their "obsession with surface effects" ... but more like "bcs Extended Spell would otherwise do litteraly nothing in most cases" :-/
Extend spell are meant to prolong spells that has a duration of 1 minute or longer. For example buffs like haste, invisibilty etc or debuffs like slow and fear. Those are really powerful spells so doing nothing ain't exactly how I see it

It is not meant to be used for short 3-4 turns debuffs, and in RAW it doesn't.
Now, imagine multiclassing a sorc with cleric, getting access to divine buffs and extend spell definitively gets worth a sorc point.
That is both matter of future ...
If i remember corectly, invisibility spell now last forewer. O_o
We do not have haste, slow or fear ...
Anyway, we obviously didnt understand each other.
I know what Prolonged spell do in Tabletop ...
I was talking here about BG-3 ... and there as far as i know, isnt much :-/
You could theoreticaly prolong Friends ... but that would mean implementation metamagic even to dicerolling UI. :-/ Wich obviously nobody worked on yet.
Simmilar to subtle spell ...
You are suppose to cast secretly with this metamagic ... in BG-3 you are just able to cast under effect of Silence.