As far as the degree of change, personally, based on what I have seen, it seems that Brawl vs Melee will be a smaller change than Melee vs 64. Based on your logic that "changing many game mechanics from wildly successful games -- in this case, SSBM -- results in a lesser quality of game," that would mean that it is in fact more probable that Brawl will surpass Melee by a larger margin than Melee did 64.
You missed one not-so-minor point: the requisite for change equaling a declining game is when the game has already achieved greatness. The original SSB was nothing close to great. It had holes all over the place, and it is quite amazing to see what HAL did with Melee from SSB64. But once you already have something that's great, changing many things -- some of which will definitely be things that MADE Melee great -- means you have to change them into things that are equally good or better. That very rarely happens. Looking at most successful long-term franchises, change is done over a longer period of time, with a couple of individual gameplay mechanics tweaked. When more than a couple are tweaked, it is usually not nearly as good. (See: Super Mario Sunshine.) It is the exception, not the rule. Based on what (admittedly little, especially for such a high-profile game, though you could easily interpret that to mean a LOT is going to be changed) evidence we have available, I have seen nothing to suggest that Brawl will be an exception. I hope it is, but I am not expecting it.
A "witty" quote proves nothing. I think that's a rather ignorant argument to make. Your (or anyone's, not to single you out in particular) inability to conceive of further improvements has no bearing on the actual metaphysical possibility of improvement. That's kind of like saying "Well, since Led Zeppelin (or whatever) is the best rock band ever, it's highly probable that anything after them in that genre will flop."
Inability to conceive of further improvements? There are a couple of potential improvements to better Melee. That's not the point, though. You yourself just stated it looks as if Brawl is going to see as many changes as Melee did from 64. My point is that when you are already dealing with a baseline of a great game, as Melee is, a significant amount of changes almost invariably results in a weaker game. This isn't some theoretical statement; history serves as proof. "If it ain't broke, don't fix it."
Halo 1 and Halo 2 had completely different physics engines. Halo 1 was considered by many to be the best console FPS ever, or behind Goldeneye. They did a complete revamp and the game became more popular.
I seriously hope you're not trying to imply Halo 2 is not similar to Halo in terms of game play. Epic Games rehauls the Unreal engine all the time; are you trying to argue that the successive sequels of the Unreal series are not similar games at heart? Because the graphics engine has been greatly improved, the gameplay differs? I think not.
lollerskates Halo only became more popular because it got online play, wtf?
I don't hold this belief that you can't change something thats great for the better.
I am struggling to think of successful, viable video game franchises that have a history of superb games and that had sequels that changed many FUNDAMENTAL gameplay mechanics and came out with an end product that was equal or superior to the previous product. Zelda, GTA, MGS, they're all quite similar. The only thing that is remotely coming to mind is the Final Fantasy series, and one can see throughout the series' history that many of the core mechanics EnixSquare toyed with were not well liked (see: Final Fantasy VIII).
My point is not that brawl will be better, but that it will still sell a ****-ton of copies even if its overhauled for casual play.
My point is that the very overhaul in question will lessen the game, not with what aim the overhaul takes place (i.e. to make it "more competitive" or "more casual"). The reviews will reflect that.