I'm not saying he wouldn't, I'm saying there's no precedent. Sakurai isn't the marketing team. I'm sure he gets final say on almost everything, but Nintendo's marketing team most likely decides things like Direct dates and when to send out certain information, and from a marketer's perspective, it's typically a very dumb idea to save your ace in the hole for a mere month before the game releases. This speculation cycle is different from previous ones, though, because we didn't even know about the game until six months before release, so they could be doing things differently than in games past.
Regarding Shulk, I'm aware that people who knew what Xenoblade was were excited he was in. People were also excited for Pikmin representation. But I'm comparing Shulk's reveal to goddamn Mega Man, or Pac-Man, or the Animal Crossing Villager, who had easily one of the biggest fanbases in the build up to Smash 4. Any way you slice it, they weren't saving the very best for the very last.