It is because the parts left out, because of out cry by the community, added a large amount of content to the game. Legion and Shadowlands borrowed power added a large amount of content to support it. Does that mean that content will be automatically less? No. It does mean that we will have to have more basic forms of content in order to have similar levels. Even the mission table is going away so we won't have the small extras that has provided over the years.
Think about how Legion would have been if Order halls and Artifacts didn't exist. Would it still have been a decent expansion? Sure. Would it have lost a large chunk of content? Yep. So Dragonflight does have some gaps in content to fill. Dragon Riding could fill that depending on how much the flesh the system out.