I don't disagree that it will become more and more prevalent as consoles march toward the future. I don't think consoles will ever be able to muster anything that is seriously
interesting however. At best you get generic looking levels pieced together from premade objects. That can be fun at its core, but it's never going to stand on its own as many mods for PC games have. Brawl's level editor is a good example of just that as well. It may be easy to use, but that comes at the cost of it being so limited that it's almost not even worth using. I don't see this changing much, as consoles simply do not have the input methods and flexibility that personal computers do. This concept may be improved, but it will remain essentially the same. Just look back at the aforementioned Excite Bike. You are doing the same thing there as you would be doing in Tony Hawk or Brawl. You choose pieces and assemble them, never straying too far from what the developers intended.
I will, however, disagree with your bullet points. a)
Many high profile PC franchises have been including map/scenario editors with their games for years and years now.They may not be built into the game's menu, but they do come included.b)
Many editors are fairly simplistic, especially if you just want to make a quick level. You can even go a step further and peak at Warcraft of Starcraft's editors. Those are quickly mastered, though tend to be more in line with that console editors are. While I certainly don't expect anyone to be able to pick up Radiant and make an awesome level in fifteen minutes, it isn't exactly rocket science either. It's very doable. You put a little more effort into these types of editors because you are literally using the tools that the developers used. You're not limited in the least.c)
Depending upon the game, this could be true. Most major titles have fairly centric fansite (or even official communities) that share such things however. Junk isn't automatically downloaded onto your system like in Brawl, but I wouldn't want it to be anyway.
Now, while mod creation is generally reserved for a niche group, I don't think that those who partake in the end result are as small as you think. Something as simple as downloading a new player skin or map is far more common than I believe you are leading on. It may be different with Half-Life, I don't know though, I was never much into that scene. Now, while I agree that user generated content will become more and more popular on consoles, I don't think that it will become grander than it has been on the PC for over a decade. Consoles are simply too limited to exceed their own boundaries. Besides, Microsoft would want you to charge everyone for any mods you make and give them a cut of the profits.