No loading screens took more than 10 seconds to load in any of my Oblivion's playthroughs on a low-end PC with a lot of mods.
I find your point useless for two reasons:
1- The loading does not take much time and it helps to keep the game stable and running smoothly. Also some cells are stored in the system's buffer, allowing you to quickly exit one to another in a couple seconds, the same time it would take for you to wait the door animation to play and open it.
2- There are too many things to render and simulate at the same time making it impossible to have a unified cell for the whole game. I am not talking only about items, but scripts, physics, animations, lighting, NPC AI packages. It would destroy your computer or console.
1.So does optimized engines. How do you think CryEngine 2 was managed to pull off what they did so early as in 2007?
2. False and not impossible. Far from. All fable games have had it. Red Dead Redemption (a big open RPG) has it. Crysis had it in big camps as well (although it's not an RPG, it has a lot of stuff anyway). Also given that these games will all be considered "old" when Skyrim comes out close to 2012, it only enhances the distinction. Your logic that no games can handle it pretty much fails here.
I think it really comes down to optimization of engines. This goes for PC/PS3/Xbox 360.
Having loading times by cell seperation is kinda bad. It feels so... outdated. So static.
But the worst part is that you can't fluently play the game. Going out from houses or dungeons with a loading screen is really bad. Otherwise, you would get this "wow" feeling coming out a dungeon with no loading screen in between. Cell seperation pretty much eliminates that "wow" feeling.
Also, cell seperation makes (for instance) enemies that chase you suddenly fade from thin air. Very immersive-breaking.
Finally, there would be no easy way to view exteriors from interiors. A few mods for Oblivion have successfully done this. If cell seperation is an absolute must (probably by performance issues from poor engine-optimization combined with too many items) then Bethesda should do what these mods have done.
I've also been wondering if it can't be designed so that the "object distance" is tied to what you can actually see. For instance, look at a house from exterior, and the objects inside it doesn't get rendered because of the house's walls blocking your sight.
This would perhaps be a solution to any performance issues related to no cell seperation? Not sure though, as I don't know anything about scripts or anything like that.