Adding in new ranged weapon types like slings and darts would probably take a bit more resources than usual,
I suppose my counter question would be, why is this more resource/effort/value for Larian than it's been of every other company that's ever made a D&D rpg? Why do they get a pass on this kind of stuff, because it's 'more effort'?
They don't, at least not in my book.
The benefit is the benefit it always is - and it's not a strictly mechanical one, but one of a feeling of completeness, of options, of freedom to choose and freedom to characterise - things which matter more n this type of game than most. Larian doesn't get a pass on that because it doesn't match the specs of what they can hack together or modify from the specs of their old game engine. At least, they don't for me.
Sorcerers missing those proficiencies reads a lot more like Larian deciding not to add them into the game, in the end.
Unfortunately, I do feel you're right. Biting off more than they can chew, over-promising and then under-delivering, is one of Larian's strongest hallmarks of game-design at this point. This game deserves better.
