Ugh.. This turned out longer than I thought.. It's better that I don't quote on this one. Long read ahead!
Exactly what aspects of the Demo suggest that Brawl will be less competitive than Melee? The game indeed has different mechanics and whatnot, but it doesn't necessarily mean that it lacks depth. Different does not equate bad (nor worse). I'm sure I don't need to say this again, but anyways, it was just a Demo, and people were only able to get a couple hours of gameplay. It took -years- of playing to make melee what it is now, depth-wise (at least, it took years for its depth to become apparent). Don't expect Brawl to be easy to master in a day or two just because you have mastered melee. A new game must have new depths to explore, else it would be just a remake of the original.
Yes, Nintendo has the means to make patches to the game, but if you take a moment to view it from their perspective, what benefit would they get from doing so? That's considering they would be free, since we don't have to pay to be able to go online, so why would they spend time figuring out if something needs balancing after the release? Again, it would only be to please the competitive community, which is the minority, and doesn't represent a big loss or gain in doing so. Don't get me wrong, though, as much as I'd love to get balance updates and whatnot, it's unlikely for them to keep working on that after the release. It is the hope of many that during this extra time they have, they make the game as balanced as possible, so that there is no need for patches. It's not entirely a lack of effort; it's just unnecessary post-release effort that I hope they make up for with the pushback.
About the variety, brawl has the same amount of moves that melee did, so let's move on to combos.. In smash, there are no "defined" combos, unlike in many other games. You just need to get creative, and the aerial combat helps that. Like I said before, with the new L-canceling, there are tons of potential new combos to discover and try out. From my point of view, it would seem that there will be more freedom than in melee in that sense. Also, from what I've read, items were banned mostly for the exploding crates, so there is a chance they may be allowed in brawl; don't count them out just yet. And about the stages, that's the best part about that nice feature called Stage Editor. If you don't have enough stages viable for competitive play, then create your own! There is no reason to complain about stages with that feature.
How am I underestimating the smartness of good players? I don't understand what do you mean by that. And by the way, creativity doesn't have limits, even if the game does have them. Let's say you do "reach the limit" and master every technique the game has to offer (which I highly doubt will happen for a long while, anyways). Does that suddenly mean that there are no more challenges? Not quite. Perhaps the game itself has no more challenges for you, but the other players do. Other players can get as good as you (or better) and beat you. You have to take in other factors besides the game mechanics, such as reflexes, strategy, etc, all of which can be improved/changed.
On the last 3 things, again, variety so far seems better than in melee in some aspects. How can you say that brawl requires less timing than melee, though? IMO, the timing used in melee is already very good and brawl should at least be the same; there are other things that can be improved. Finally, about the speed.. I hope you're not one of those people still complaining about the wavedash -.-. Did you actually play the Demo? Or do you base your comment on the vids? According to gimpy, they figured out the l-cancel until the last day, so most videos (if not all) are not using l-cancelling at all. The reason you see everything going "slower" is because you're most probably used to battles where the wavedash and l-cancel are abused, thus making the game look a lot faster. Once people get the hang of the new l-canceling, I'm very sure that the game will start to look faster once more.
And one last thing.. Even if all that you say were true and brawl lacked the timing, speed and variety that you want... People would still play it! People would still play it competitively too! Why? Because as stated before, this game is lots of fun, and it is essentially the same as melee, but with new features and such. The game will still be competitive, given that you can eliminate luck factors almost completely, thus, allowing skill and strategy to speak for themselves. Even if it weren't the perfect game for whatever reason, it would still be a great game and would have a big competitive community, even if some people that don't want to adapt decide to get left out. Like you said, more people will take their place, but I don't see the community dieing that easily. Besides, people would go back to it sooner or later. Good enough proof for that is that a good amount of people still play SMB64, which doesn't nearly have said depth.
-Deep breath-
Bottom line... Cheer up! This game will be awesome! For all we've seen, whether or not you'll like it is not up to the creators at this point, it's up to you. If you'd like a piece of advise, if you're being negative, things will must likely end up negative as well (or at least, it will seem that way for you); if you try to keep a positive mind, you'll be able to enjoy things more, even if they're not as good as they could be. It's all up to you, anyways.