This = failure, because I'm essentially forced to do this.
That's the only real issue I have with it, operative word being "forced" - as in forced to conform to a predetermined path. That's contradictory to "non-linear," "open-ended" and "sandbox," terms traditionally used to describe the Elder Scrolls games.
That was one thing I enjoyed about TES over another game series I really enjoyed: Final Fantasy. I was a little frustrated the first time I played Final Fantasy 7. That was my first intro to that series, and I had already played Daggerfall and Arena beforehand. I found it really annoying that you were forced to progress in a specific manner with the story. In other words, it was linear, or "scripted." You couldn't really just go off and explore and do your own thing, other than very limited exploration of restricted areas (using cliffs and mountains and other objects to keep you in a small area).
However, the Elder Scrolls have always been non-linear and open-ended, and not only was the player free to explore in any manner and do anything he or she wanted in any order, but it was encouraged and even advertised as part of the game's features.
That said, I do realize that true open-ended and non-linear exploration is going to lead to a problem that has plagued that last couple of TES games, which is that if you explore too much too fast, you'll end up burning through all the content with very little good loot to show for it and no challenges left. I can see the virtue of dungeon level locking, I'm just not sure it's the best answer. And people need to be careful using "realistic" with a fantasy game. So that bear cave you explored at level 10 now suddenly has daedra in it at level 50? So? How is that not "realistic?" You cleared out the cave. Should it now remain empty for all eternity? Somebody else moved in, and it just happens to be a much stronger somebody else.
I guess I just don't see how respawning creatures scaled to the player's level is that bad. I do think it's a good idea to have content that is simply too hard for the player at the beginning however. Thus I think having set minimum levels for dungeons would be a better solution. For example, Dungeon A might be for level 10 and up, and every time you visit, the creatures are scaled to your level but never lower than 10. Dungeon B is a level 50 and up dungeon, so that every time you visit it, the creatures will also scale to your level, but never lower than level 50. Thus, it is too hard for a level 20, but still provides content for a level 75.
What happens if you choose to the main quest first and then go exploring? Now suddenly half the dungeons in the game might be trivial and pointless to explore, because you already "missed your chance." Missing your chance sure doesn't sound non-linear to me.