It's a shame indeed, but that's how most of the big studios work these days. I can't think of any modern FPS or a similar game that comes with a proper level editor. I suppose it would take quite a lot of effort from the studio to deliver such an editor, and I doubt that many people would be interested. Games today aren't supposed to last anyway or meant to be replayed time after time. You complete a game, maybe give it one more play through, and then you forget it and move on to the next game. Of course there are exceptions, as always.
But yeah, it's a nice thought.With some tweaks Thi4f could have been a foundation for something great indeed. But since they never even released any more DLC after the Bank Heist mission, I guess it's clear that Squeenix had no further plans for the game. The Bank Heist was such a good mission though, better than most of the missions in the actual game. I would have been happy to pay a few bucks for more missions like that.