I believe it was an article/interview posted, but I will get the link later. It was quite some time ago when the Dojo was almost just released to the public.
Also, You have no idea what Miyamoto will do. And if he does continue it, it won't be for another five or six years. Sakurai's age has nothing to do with him being a director.
Sakurai knows it's making money. Also, again, there is a reason he left HAL Laboratory. People, like you, with that general attitude of money money money, give me sequels and what not, caused him to leave. Sakurai isn't the type of man who likes sequel after sequel. Oh yea, Look at Mario Party. Guess how many sequels it has?
Eight. Are all eight good? No. Take a look at the ratings. The first 3 were successful, then 4-8 just got repetitive. Do you want Brawl to be the same way? After all, it's all for the cash, not the game at all.
http://en.wikipedia.org/wiki/Masahiro_Sakurai
"It was tough for me to see that every time I made a new game, people automatically assumed that a sequel was coming," said Masahiro Sakurai on August 26, 2003, in an interview with Nintendo Dream, two weeks after his resignation from HAL Laboratory. "Even if it's a sequel, lots of people have to give their all to make a game, but some people think the sequel process happens naturally."
Edit: Aha. He didn't directly say it, but it's implied.
After Melee, Sakurai thought
he was done with the Smash Brothers series, until at E3 2005 Nintendo asked him to work on a 3rd installment. He wasn't sure he was
even going to do it at first, until he realized that if he didn't direct the game, it would be handed off to someone else, and Sakurai didn't want to see the community let down. That is why he is currently working on Brawl.
But what about after that? Will there be anymore Smash Brothers games? He has already said that
he is sick of people assuming that there will always be sequels to certain games. But he has also acknowledged how important Smash Brothers games are to the fans.
http://blogs.ign.com/Blogs/Comments.aspx?blog=WindWaker99&entryid=47663
Basically, Very unlikely.