Let's try a though experiment.
Let's say a new Smash releases and there are cuts.
But these five characters stay.


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
What do you think would be the developer's justifications for keeping these five? They don't all have to have the same reason.
Then they'll probably keep cuts to a minimum in general? And I'd expect many cuts being from third party companies not agreeing with certain licencing stuff.
Young Link has potential of a Majora's Mask total revamp. I could see them keep him, and add Skull Kid. But cut Toon Link maybe.
Pichu? A clone of the most popular or at least well known Pokemon, or rather just the mascot. That's probably excusable. Maybe they'll include the likes of Pichu and Dr.Mario to the branch of Echo Fighters.
Plant might just be them preferring uniqueness and weirdness. I dunno. I expect this character cut honestly. But they might just keep him because they like weirdos.
Incineroar has no reason to be cut whatsoever in my opinion anyway. Controversial character maybe, but who cares? He's popular. When it comes to Pokemon suitable for a fighting game, this is an extremely solid choice.
And Corrin? Maybe because all the Fire Emblem character are being kept relevant cause of Engage and Heroes. Intelligent Systems tries hard to keep their most well known characters as accessible as possible. Corrin was chosen amongst the first batch of Emblems in Engage. He was chosen before Hector, Chrom, Robin... now that's a thing. I can see it.
Cuts I think would happen if these stay are amongst the likes of Ice Climbers, Wii Fit Trainer, probably Toon Link since Young Link stays (not my choice out-of the two but yeah), and a few third party characters of which I hope Steve is amongst.