Comparisons to Elite, even say X-Rebirth, are fair enough when looking at the bulletin points of features but NMS is an insult to all of them. There is no depth to NMS where there is in the others, everything it does it does light. The reason I say it's a naive comparison is because the weak similarities are only on the surface and entirely ignore the design and technical intricacy that allow for more complex gameplay, gameplay that is different whether you acknowledge it or not.
On top of what others have said a single example on how they differ; the way CIG are tackling physics. Before they started development the system they've created had never been successfully done before and while others are now adopting it it's still not on the same scale. Seperate physics systems within physics systems, all 'interactable' with each other on a large scale.. that's what allows them to have ships flying around with people walking around inside them alongside other ships with other people (as simple as that sounds, it's not and thus why all previous attempts had essentially failed), it's that level of technical depth where these games vary massively and it comes through in how the gameplay actually functions such as breaching the side of a ship while it's in motion and flying inside it to engage in person Vs person combat without the need of scripted events or instancing while maintaining the same physics system (ever tried to physically ride on a vehicle in something like Arma?). Other space sims didn't just choose to not have that aspect of gameplay, they weren't/aren't capable of it. As said some games are beginning to introduce it now but it's rudimentary and often at the expense of other aspects, CIG are attempting to reach that level with all areas of all features of the game.
Star Citizen isn't doing anything different, or new more appropriately, in the sense of having spaceships, planets, mining or combat etc. The difference is in how it is doing those things and while that seems to be irrelevant to you for some reason it's what governs how the game is played & feels, what is possible and what isn't. The pursuit of a greater level of technical depth & general quality are what justifies the cost & time and will also be what potentially ruins them if they can't achieve it, they aren't just failing to do what's already been done.
By that simple mindset space sims have had the 4X list of features for decades and therefore every game you've named is pointless, we may as well have stopped with X: Beyond the Frontier. Why have developers and publishers shelled out hundreds of millions of dollars between them to create more advanced game engines and assets?
*edit* For the record I honestly don't believe they'll achieve what they've claimed, I think they'll land somewhere between current specs and where they're aiming and every step they take towards the latter will potentially cause more and more issues with gameplay in the released product (physics lag etc).