Mother doesn't have SSB's sales potential
Granted, Mother is no where near as popular as Super Smash Bros.
Sigh. Yes, I know. However, I was specifically responding to your skepticism that Sakurai would ultimately decide whether there's another installment. Objectively speaking, we know this has happened before and Iwata respected Itoi's decision to end the Mother series.
In this particular situation, we have the president of the company stating (many years ago) he didn't feel Super Smash Bros. could continue without the series creator's help, and the series creator stating he doesn't know if he'll make another installment. Reading between the lines, it doesn't sound like Iwata has faith in anyone else to direct the series, possibly due to the commitment necessary to do it justice. Couple that with Nintendo's habit of outright canceling games that do not meet certain standards and I don't believe it's too farfetched to think the series ends here if Sakurai chooses not to be involved at all.
I agree that Nintendo probably doesn't
want to give up the series due to its popularity and profitability. However, we've seen many companies try to emulate the style and concept and they don't touch Super Smash Bros., like not even remotely. Super Smash Bros. is a big investment for Nintendo in terms of both development and marketing cost, and I can't imagine it's too eager to leave the series in incapable hands. Iwata sees Sakurai as the heart and soul of the series. To expand on what I said previously, if Nintendo (or Iwata, specifically) could get Sakurai to oversee a sequel and act more as more of a supervisor to the new director, guiding and prepping this person to inherit the series or so to speak, that would be a compromise I could see.
Remember that Brawl was announced without Sakurai's knowledge. They then asked him to work on the game but had he declined do you think they would have just cancelled it? I'm assuming they were making an SSB game, period, and if Sakurai wasn't on board they would have just made it without him.
According to the
Super Smash Bros. Brawl's Iwata Asks:
Iwata: Well, I had considered what I would do if you turned me down and decided that I would need to take the existing Smash Bros. title, Super Smash Bros. Melee for the Nintendo GameCube, and try to make it Wi-Fi capable while preserving as much balanced game play as possible in the event you didn’t want to get involved. Maybe it’s more appropriate to say that I realized we wouldn’t be able to add any new elements to the game without your help and I think I said as much when we discussed it at the hotel. It wasn’t right, but you might even say I used it as a threat of sorts.
And there's your answer. We wouldn't have Brawl; we would have had Melee Online. Considering Melee's religious following, that would have been enough for some people.