Point is, if we're going to use this analogy to describe ED's content then it's worth considering that a very small increase in the "depth" will yield a gigantic increase in "volume".
Basically, if ED is "an inch deep" and has a "volume of content" equivalent to 26 olympic swimming pools then simply making it two inches deep will double the "volume of content" to 52 swimming pools.
No matter the volume, you still can't swim in a 1inch swimming pool, and you can't do many other activities either, because it's too limited. In fact, having such volume of water used for basically nothing meaningful is a big waste.
Looking at ED since launch, we've had engineering, planetary landing and legs which have all added to all facets of the game and, thus, multiplied the depth of the game.
That's where you are wrong, those features didn't make it deeper, only larger, so now, we have a 2mile long swimming pool, but with only 1inch of water.
While the analogy is accurate, I prefer to think ED is made of beautiful, promising foundations with nothing on top of it. That's what I felt with every aspect of the game ever since I started to play.
Combat is about pewpew random dudes in a random place (space or ground). Missions that existed decades ago already in others games. In fact, Freelancer had slightly more varied combat missions, 20years ago (attack/defence of satellite/outposts for example).
Powerplay is just siding with a faction, and its leader. Both are protected by plot armor, and NOTHING will ever come out of it. PP was introduced 7years ago, and literally nothing changed to the status quo.
Despite Braben being known for his work on procedrual generation and the Stellar forge, the plants and even rocks in exploration are all from a limited pool of assets, and there is little sense of discovery after your first 10th discovery of the concha labiata (and giggling about it), this time in red instead of green