It's not "end Smash on a good note", because good is indeed subjective. It's "end Smash on a climax", like with a potential headliner. Like it were a story. These characters, Olimar, Shulk, Corrin/Bayo, Incineroar/Ken, Byleth... whether they're "good" is up to you. But they're not headliners, they're not the "big guns".
And that's not just because Nintendo doesn't feel like ending on a bang. If it were, then, sure, a change could happen whenever. It's a marketing decision. Smash isn't a story, it's a business. With base, you want to start with a bang, make a good first impression, then keep the hype rolling. With DLC, your install base is gradually going to shrink, you want to use the bigger characters earlier on to move more of the passes. How many people do you think would buy the pass based on the sixth character almost two years later as opposed to those within the first half?
You want to attract as many eyes as possible. It's no coincidence every Smash-attended E3, the event with the largest audience, has reliably had a heavy hitter present. Most patterns are indeed fantasy or coincidence, but those that revolve around marketing often have some credence to them. To that end, it will be very interesting to see who FP7 turns out to be... given they were presumably planned as an E3 reveal.
I think Ryu would've been second most surprising had he not been leaked to the point of being playable prior to his reveal.
And Mewtwo was arguably the most demanded character when he was revealed, so I have a hard time putting Bayo ahead of him in terms of excitement.