The thing people don't always keep in mind about Super Smash Bros. as a series, is that its basically one huge advertisement. It is used to introduce particular series to players who haven't played it before, and celebrate the company's history as a whole.
Had it not been for Smash Bros., I would never have played Metal Gear Solid (which is now one of my favorite game series of all time), The Legend of Zelda, Metroid, Kid Icarus: Uprising, and I really wouldn't have an interest in beginning to play Fire Emblem once Awakening is released in North America.
So really, if advertisement is an issue, this is the game to put a character in. However, that doesn't mean other characters won't get in, as recognizing popular games and their cast is important as well, which also advertises other games. Xenoblade Chronicles, Golden Sun, and Sin & Punishment all fall under this category (Xenoblade more recently than the other two obviously) so it is quite possible that Nintendo will decide to build off of the popularity of Shulk and his game, or Isaac and his, or even both. Saki doesn't have this advantage, as his games were not as successful or widespread if you will. Basically, Shulk leads here.
Overall, here is how we can look at it. According to an article I saw in some other thread, Sakurai stated that while he wants to make the game balanced, he isn't going to let that completely limit him on how many characters he added. However, this isn't exactly what he said at the start, which is that they won't add as many characters.
So, if there are going to be less characters added than the previous games, that means we will see anywhere between 17 (I think were added into Brawl, not counting them as mere replacements) and I would say roughly 10. So lets go with a hypothetical 15 characters.
We can have: Ridley, Mewtwo, K. Rool, Palutena, Mega Man, Pac-Man, Takamaru, 2nd retro/WTF. That only takes 8 slots, and thats basically a majority of the most popular choices in this point in time.
Characters that remain that are notable include: Isaac, Shulk, Saki, Little Mac, Mario rep, Zelda Rep, Roy, Chrom, Starfy, etc...(you get the idea).
So look at it this way, basically, even after all of the most popular choices are added, there is more than enough wiggle room for other characters to be in, allowing all three of the blonde warriors to be Smash Brothers. Wonderful isn't it?
I think i'm going to further analyze the possibilities that the amount of roster spots give us, combined with the fact that there were other characters that were meant to be in Brawl in the first place. I may make a blog about it soon.
Sorry about the Wall O Text, but it just kind of kept coming the more I typed. Don't always consider the roster as tiny and cramped. Remember the possibilities!