It's the nature of games trying new concepts. There's so many new problems to solve ('What if a planet generates a cave too deep to get out off?', 'How do we get AI to navigate this procedurally generated, complex terrain?' etc) that it is inevitable it would be much closer to being a tech demo than to realizing the full potential of the concept. So much of the time and effort that would be put into making the game fun would have been put instead into making the concept work.
I don't think they cut any content, as much of the content they spoke of probably never existed. I'd wager that they had to narrow their scope to match what they thought they could accomplish as development progressed, which is common in games. Hello Games' mistake (or perhaps not; the game did sell well) was to increase the promises as time went on, instead of reigning them in.
Like it used Spore, at some point a game will use No Man's Sky as part of a more solid game. That might even happen in patches for this game.