Yeah.. I still think you are minimizing the amount of stuff it could turn into.
Now start thinking of character histories and saving all of that data. I'm not talking about just basic ship info.. things that could affect dialog, story outcomes, or items acquired. So like, character interactions from damaged by, where, and when... to like what conversations have been had in what order. Current and past locations.. Upgrade history, etc.
And it's not just that all of this can be stored in a variable. Big whoop, we know it can be done. The point is keeping track of it all. Doing a full on sandbox with string and int variables would be a ludicrous amount of work with the way FSO is currently constrained to saving data and passing it between missions. It would take document after document to even remember which bit stored what info and for which character. Not to mention the SEXPing required to even do something with all of that info in any and every mission that's part of the "campaign".
I mean, we aren't talking about variable tracking for some BP dialog tree. We are talking about tracking full on sandbox data for any number of systems, planets, nodes, characters, upgrades, ships, and whatever else needs to be packed into a space sandbox to make it interesting. It's not that it can't be done. It's that trying to use any form of FSO variable-SEXP system to make a sandbox is like trying to build a real house out of Legos. I'm sure someone could do it, but it would be a stupid amount of work.. and Legos just weren't ever meant for that.