There are definitely scenarios where K. Rool doesn't get in. The biggest worries to me personally are Sakurai disliking the character/the series, him being held back for next game, or Dixie being preplanned for whatever reason and them not wanting two DK characters. Those could all legitimately happen, and we'd never know until after the fact.
The most talked-about scenarios, on the other hand, aren't worrisome at all. "He's popular, but popularity doesn't matter, it's a suggestion box, not a poll" is really a hollow criticism, since the medium of DLC mandates that a character be popular so they actually sell, and K. Rool has fans writing in excellent substantive reasons to put him in anyway. This means that, no, Chorus Kids are not going to be plopped out there ahead of K. Rool or Isaac as "your fan favorite ballot selection," regardless of how onboard you were with them back when Gematsu reigned supreme. Even if DLC is preplanned, there's been precedent of unplanned newcomers being added due to fan polls with Sonic in Brawl, and although K. Rool may not reach those heights, if there's going to be any unplanned standouts, he has an amazing shot at being one.
The other one is "Dixie is easier to make, so she's more likely." That's easy to see through as an anti-K. Rool talking point, because if ease of development were really that big a problem that a Top 3 character would be snubbed for a Top 20 one (estimates), why is it that that argument only ever comes up at K. Rool's expense? Why are we bothering talking about remotely physically unique characters like Isaac, Inkling or all those third parties if they're such a crushing burden to design? Just throw in Dixie, Galacta Knight and Louie and call it a day, right?
Every character gets built from the ground up, according to Sakurai, so even if K. Rool doesn't have a pre-existing moveset (to our knowledge), he'd not be ridiculously hard to make compared to a semi-clone. His movesets write themselves and he has no crazy gimmick to create difficulties, in any case.