I have slight doubts that this will be the last Smash Bros., but sadly, everything must come to an end sooner or later, so I wouldn't be disappointed if it were indeed the last game. I would play it for years, I'm sure of it. I don't know what kind of gameplay the next SSB would have though as I don't even have a Wii and know almost nothing about THAT kind of gameplay, so I'm kinda lost there. And I also don't have a slight idea what characters the developers would put in since it all depends on what they do with Brawl.
Well, other than video game consoles developing to virtual reality, how are they going to continue any of the big three lines? The answer is that as long as they want money, they can find things to improve. Not everything may be as big a jump as from the PS to the PS2, but everything will always have room for improvement (at least as long as you are alive, anyways), even if it is just investing billions of dollars into graphics, or memory, better online play, whatever. The same goes for the biggest video game series Nintendo grasps.
If they stopped making the games, it probably would not be because they ran out of ideas, more likely because the games stopped selling well or because Nintendo went out of business.
I personally am not so scared at the thought of not getting another game after Brawl (although one more would be good). Look how long Melee has lasted us, and predict how much longer Brawl will. We can continue playing both games both competitively and casually with our friends until we grow out of this part of our lives. Finally, it is true that unless something goes horrendously (sp?) wrong, we will almost definitely be recieving more games after Brawl, so don't even bother worrying.