When people bring up that Shulk and Olimar were the final reveals of the last game, I think that's not really looking at the entire picture of why people believe ending on "just" Ken and Incineroar would be kind of weird and out of order with the rest of Ultimate's news.
With both Brawl and Smash 4, there was a pretty consistent drip feed of newcomers at a fairly regular pace. Newcomers weren't front loaded, and one a pace was set for info, that pace was kept right up until release. So, while Olimar and Shulk were not massive hype machines, they were released at the same pace that announcements had always been (let's ignore the Sonic delay, because that messed the pace up)
Take Smash 4, for example. Once hype and PR really started to ramp up in 2014, with the release of Rosalina, from there, there was a pretty decent pace of one newcomer every few months. E3 obviously had more, but once E3 ended, we were right back to 1 character every few months right up until Smash 4 Wii U released. So even if it technically ended on Shulk, right before that we had Robin/Lucina, before that Miis/Pac Man/Palutena/Dark Pit, before that Greninja, before that Little Mac, etc, etc. And they were a couple months apart from each other at a pretty consistent pace.
What the "box theory" proposes, though, is that Ultimate's reveal cycle was very heavy in the front. Not counting the March tease, E3 gave us 3 newcomers and all vets returning, which is massive news, but expected for E3. The pace was then set with the next reveal, a few months later, when we got 5 newcomers in one single Direct. And then a month later, we got another newcomer, in a Nintendo general direct. Which makes sense, since it's not a Smash Direct and the reveal tied into Animal Crossing Switch.
But now, if there's only two reveals left, the pace of Smash reveals hits an abrupt halt where the next 3 months give us nothing more than 2 new characters
We already know all the stages. We already know the total number of music. There is barely enough to fill a massive pre-release blowout like Smash 4 had. And that's something they'll obviously want to do because the 50 fact Direct for Smash 4 was amazing. After keeping a fairly nice pace of announcements, they'll just dry up for 3 months for some reason?
So the pace of Ultimate announcements would be backwards to how Smash, and really, video games in general, are announced by going out with a smaller amount of content then had been expected by the pace set by earlier announcements. It feels jarring and not really Sakurai's style, either.
What I believe the situation is: Yes, the box art has room for Incineroar and Ken. Both will be on the box. But the box will start hit warehouses in late October, and if the box had the full roster on it, that'd be a very easy and obvious place to leak it. Rando Best Buy/Amazon/Walmart/Target warehouse employee isn't going to care about leaking the entire roster on 4chan or Reddit or wherever. So my thought is that the roster on the box is the roster, as it will be officially know in late October. This was done so the boxes could be produced without leaking the entire roster, since putting the entire roster on a box with months long lead up with multiple chances to leak seems like a terrible idea.
So Ken/Incineroar will be announced sometime in late September, early October. In a Smash mini-Direct, if I had to guess. Remember, this Direct was delayed a week, so an announcement in late September/Early October isn't as close to Isabelle as it seems. And then in early/mid November, there will be one last massive Smash 45 minute crazy Direct where everything else is revealed, including the final roster that will ship with the game, which is likely 2-3 more characters and 2-3 more echoes.
