I have thought of this before, on several ocassions. However, after really thinking about it, I believe that the controls wouldn't be too hard for some, but it would be hard for others. I wouldn't be able to get used to the jumping thing for a long time, due to the fact that I use a classic controller. A classic controller uses a completely seperate button for jumping than the analog sticks (I have no idea about other controlls). Therefore, even if it die, I wouldn't really buy the game.
Following what Ember Incubus said, I'm sure that the Final Smash would probably be considered again for the next Super Smash Bros. game. Although, if it didn't make another appearance, it wouldn't be a big loss for the series in my opinion. It wouldn't be a big loss because I usually have all items switched off for everything except having friends around for brawling and on the single player mode games, so the SSE Classic, All Star Mode...etc.
Another reason for not making a Super Smash Bros. onto the DS is that the DS has an extremely small screen. Thus making it extremely hard to see small items such as Bombs, stickers, trophies and Pokeballs...etc. The screen would have to be even bigger than that of the DS-i XL if Nintendo did decide that they wanted to release such a game for the DS. So I think that it would make everyone have to buy an even larger DS, causing great loses made by the game.
The final point being as someone else said, the DS has not got the greatest memory, therefore a lot of characters would have to be dropped along with many items, stages and game modes. I think the only way they would be able to is to remake the original Super Sash Bros. onto the DS, as that game was very small in comparison of the newer versions. I also agree with what the same poster said that Super Smash Bros. is a game in which quantity is just as important as quality.
I don't agree that it would only allow for single player battling for a majority of the time though, as I'd find that if you took you DS to your friends houses and such, they would be likely to have their DS and game with them, thus letting you battle. If you wanted to have more people playing you could ask for several friends to come around to your house, which would let you battle against 3 others. However, most people would just see the main console Super Smash Bros. games to be a better choice, and wouldn't bother with the DS version.