I want to agree with the people saying that, even if they don't add K. Rool to Smash, Nintendo will still bring him back to the mainline DK series and Mario spin-off games because they finally realize how popular he is, but I'm not sure it's fair for us to assume that. We obviously caught their attention with the ballot, as evidenced by the Mii costume, but in the almost three years since then, they really haven't thrown us any bones except for K. Rool Street in Super Mario Odyssey. Neither K. Rool nor the Kremlings made any appearance at all in Mario Party: Star Rush, Mario Sports Super Stars, Mario Kart 8 Deluxe, Mario Party: The Top 100, the DKC: Tropical Freeze port, Mario Tennis Aces, or, perhaps most concerning, the Mario & Rabbids DK Adventure DLC. And even though we got K. Rool Street in Super Mario Odyssey (and even though I was unabashedly excited when I saw it), that was only one reference amid a sprawling city of references to other DKC characters who haven't appeared in years, several of whom (the animal buddies) did appear in the Mario & Rabbids DLC.
At this point, while I would be heartbroken to see K. Rool as an assist trophy instead of a playable character, I think I would rather have that than have him not appear at all. Adding him as an AT would suggest to me that, even though they decided to go in another direction with playable characters (for whatever reason), they had at least heard the fan outcry for K. Rool and wanted to acknowledge it in some way. Failing to add him as either a fighter or an AT would suggest to me that, almost three years after the ballot ended, Nintendo still doesn't realize how popular he is (or, worse, they do realize it and refuse to bring him back anyway). So if he doesn't make it into Smash in some capacity, I'm not sure we can count on Nintendo bringing him back any time soon.
...but that's only if he doesn't make it into Smash. I still think this is the best chance he's ever had to get in, and if he does get in, then I think that the floodgates will open, and we'll be seeing a whole lot more of him in the future. (And all will be right with the world.)