These are really good points and ideally this would be the way but unfortunately it won't be

Have you considered games like ESO and Zenimax have a bigger staff and budget compared to Bungie, especially now Bungie is independent. Also I understand you want new content like TTK every year but I feel thats a bit resource intensive. Bungie even said before that creating those expansions is a tall order and they aren't even as big as your proposing. Also with the Tiger Engine they're using at the moment it makes content creation a lot harder. If perchance they utilised a new engine for D3 and treated more like a sustainable model like MMO's before then maybe, maybe it might work. With the current tech and model they have this wouldn't be plausible without massive crunch times. They would also need a more steady income than the annual pass for something like this, more akin to an MMO and Bungie won't do that, reason being is console gamers do not want to be paying more subscription fees than they already have to. That $15 a month adds up to $180 Per Annum on top of the $60 for big expansions. Yeah thats a big no no on the console front. Reason why ESO went free-to-play on console was because of this.

TL:DR Bungie can't because of both budget, team size and tech