There is no endgame in a sandbox. There is no raid. There are no loot drops. There won't be bigger and bigger dreadnaughts that require higher and higher level components. People are playing that are perfectly content and enjoying what they do in Asps and Cobras, or less. What is their "endgame"? Content will be added over time.
Currently Elite dangerousn IS NOT a sandbox, it is a playpen with 4 simple toys that you can only use in a few ways.
A sandbox would imply that you had the materials and tools to CREATE content ie. join together and build stations/bases or own systems, create player groups that fight/ally/interact/vie for assets amongst each other (or even for lone wolves to have any sort of impact on the universe)
In it's current form Elite simply let's you play with 4 toys over and over (great if you want to use your imagination and imagine what you're doing has some form of impact) which gets very repetitive after a few weeks.
lets examine the toys:
1) exploring: you can go to systems and scan astronomical bodies and sell the data. There is limited variation, stars, moons, asteroid belts, rings, black holes, pulsars which in effect are just the same thing with a different representation because you cannot really interact with any of them in a MEANINGFUL way (aside from mining asteroids and rings which gets very boring very quickly, and fuel scooping stars or dying by flying into them or black holes)
Once you've seen the limited number of things there's no more reason to continue (no meaningful way to use or interact with them)
2) mining: you can mine asteroids and rings. The process is simple and very boring (use mining laser, scoop fragments, refine, repeat. Very limited.
3) bounty hunting: scan and kill npcs (once you learn basic dogfighting you can easily destroy the hardest challenge In the game, an elite anaconda, with little effort)
OR you can search for players with bounties. This has the most "potential" for longer term challenge and play except for the problem of consequence free disconnecting during combat (to escape death) and the fact that players can simply swap to solo/group and avoid bounty hunters (pay off their bounty or simply go to another system stealthily without the bounty hungry being able to follow or track them as they're in solo mode.
This could be long term content but it suffers from the game systems and mechanics.
4) trading: buy and sell commodities or rares. Find the lowest price and sell at the highest price. Extremely repetitive as you're basically looking at the commodities list and then undocking-jumping-docking and looking at the commodities list.
this also could be spiced up with player interaction but sadly there's no incentive to risk your cargo in open when you could be safe from the only true risk in this game, the other commanders, by going into solo or group mode.
trading is therefore extremely limited in scope and variety, same thing over with little variation.
4) pirating: easy enough against NPCs thought that becomes repetitive once you learn the basics.
Against players it is very tricky as they can simply go solo or disconnect.
again, more potential for longevity and variety (due to player interaction) but is held back by game systems and mechanics.
5) reputation grinding/system flipping: currently not working as intended due to bugs and/or unfinished mechanics, tediously doing same delivery missions or kill npcs over and over with little to no variation.
and that's it.
no player created conflict or content because we don't have the sand (interactive game elements) or shovels/spades (mechanics such as guilds/corporations, ability to build or own assets/stations.
no long term player made content (true "endgame" for a game of this scale)
no ability to truly influence anything.
its is most certainly "an inch deep" in it's current form and will remain so unless more mechanics for player interaction and development are given.
elite dangerous IS NOT a sandbox, it is an open universe spaceflight simulator with 4 basic roles to play (each with very limited play options due to mechanics and system either missing or not planned)