Imagine a world where Horse Armor never happened and people just made the whole game and then a Part 2 as a whole new game later, instead of indefinitely selling tiny bits for a game forever.
Well, didn't we lived in that world?
But in the same time, I understand the burden for studios. Making a game cost so much and need to much times, that it's not possible anymore.
Would they make extension like they were in the 90s, they should starts working on it one year before the game release if they want it to be available one year after the said release. But this lead to two issues:
Firstly they would works on a game that isn't finalized, what imply that any correction would need to be done twice
(once for the game, once for the extension). Secondly, seen how a part of the community react, they would quickly be shamed because they should have included this in the initial release.
Not that it excuse the price, and even less the overpriced purely cosmetic extension, just that we can't anymore expect games to works like they did when 10 peoples were enough to make one in less than one year.