Just like Lucas couldn't/wouldn't secure rights to Flash Gordon, we need an exciting new franchise to get behind (and it won't be from Cameron either). I'm optimistic that can happen, but I think it will be "underground" global phenomena rather than overground corporate shilled-to-death. The Matrix came close IMO, but no cigar.
I don't think I agree with you about Cameron.
Think of it this way: Sci-fi epic is released, becomes a must see event by pushing visual effects to the next level, and it ends up being the highest grossing film of all time. Critical reception is mostly positive, it is nominated for many Oscars including Best Picture which it loses, but many people don't believe in the hype. Granted, the film builds much of its content from pre-existing properties.
Now stop. What film have I described? Star Wars. I know many dislike Avatar, but Star Wars wasn't a sacred cow when it was released either. Now I'm sure most of you discovered Star Wars when you were kids right? Well I doubt you could find a kid that has seen Avatar and didn't like it. Also, Avatar 2 and 3 haven't even come out yet. So we should wait before we pass too much judgement.
As a side note, I think everyone should be looking forward to Avatar 2. If you've seen Aliens, Terminator 2, or, hell the Empire Strikes Back, you should know what kind of movie we'll get.