(not going to read all of this thread, but...)
At a basic sense, I always took sandbox to mean you're provided with some basic building blocks and tools (and I mean that metaphorically) and you then make out of the game what you want. There is no pre-defined "end game", you're free to muck about doing what you want. No script, etc.
In that basic sense, you would have to concede ED is a sandbox. My problem is, it's not a very engaging sandbox. Let me make a Minecraft analogy, and please bear with me... with MC, the interaction between the various aspects of the game - the mobs, the game physics, the blocks, the items and the mechanics around crafting, you get a very rich, practically infinitesimal number of combinations of activities. The way MC has endured over time is testament to this.
Now ED doesn't have to have the same mechanics; no-one is wanting to see crafting tables, magic or mining in ED... oh... err... hold on...
The problem is, the mechanics of ED aren't that deep to provide much life to an open, sandbox-style play. You've got mining, trading, pirating, bounty hunting or discovering as gameplay styles. But each of them is pretty formulaic, bar tweaking their effectiveness with upgrades... a game-wide mechanic that is welcome, but it alone is not really enough to provide long-term engagement (well, not to everyone).
The only honest answer I can say is, I'm not feeling the "hook" at the moment. There's not enough depth to what's there to provide the variety of gameplay. Maybe it does need some scripted gameplay to make it interesting. Maybe we just need to wait and see what Frontier have up their sleeve in terms of additional facets. At the moment, I log on each night, try to push to find something exciting happen, and just end up burning away another hour of my time doing the same old same old.