Ok i agree with most of what you said.
However consider this.
Bethesda creates a modding system like little big planets.
Console users have a "creation mode" which allows them to make there own quest, own NPCs, gear dungeons etc.
And they distribute it LBG style and allow all players acess it through a network.
I think this idea might work, altough creating somthing like this for an RPG isn't the same as for platformer.
I had to go look up Little Big Planet, because I don't have a PS3 and I've never played the game. As far as what I can tell, although they do indeed allow user-generated levels to be swapped, the nature of the game seems to me to be (to some degree) simple enough that there's really no danger of people downloading buggy levels that crash their console, or of downloading inappropriate content.
When I played Spore, though, although they policed the Sporm pretty well, there were a ridiculous amount of crappy creations that flooded the Sporepedia (their model archive) and it got pretty tiring looking for decent models amidst all the crud. And I do mean crud; after all, the game was played by all kinds of people and all ages, from eight to eighty. You can't imagine how many things you'd come across that were a few simple shapes and no paint.
User-generated content can be really great, and it can really svck. I certainly prefer having mods available - that's why I got the PC version even though I have a 360 - but I can certainly see why Sony and MIcrosoft are wary of having their user experience degraded with cruddy, buggy user-created mods stuffed to the gills with stuff that would piss off too many teenagers' parents.