Aside from the fact that Metroid's pretty poorly represented in the SSB roster, only having a single person representing the series despite the fact that it's tripled in size since the N64 days, I feel like Ridley's playable because, based on what we've seen, it's either that or he's a slow-moving boss that is roughly the size of a larger playable character like Bowser or Rosalina.
I feel like Sakurai would have to be intentionally oblivious to not be aware of the character's fanbase. Not including him in Melee was understandable, since Super Metroid was still the latest game at the time, and his absence from Brawl was at least made up for by giving him two boss battles.
However, if he's not playable in this game, and instead, he's a glorified stage hazard, there isn't any particular justification for it, or anything that makes up for it. Sakurai would basically be giving Metroid the shaft despite knowing that there are a fairly significant amount of people who have wanted to see Ridley playable in Smash Bros. since Melee, in addition to the people who wanted to see Dark Samus playable (or the hunters from the various Prime games, but with the exception of Sylux, they were one-offs, so they've never seemed particularly likely).
Admittedly, I don't feel particularly sure about his chances; I know that Metroid isn't exactly popular in Japan. However, with the information that we've been given to go off of, I feel that it's more likely for him to be a playable character with Sakurai trolling about the boss thing (which isn't exactly unusual; he trolled the viewers multiple times in that Direct alone) than it is for him to be the smallest boss that we've seen who doesn't seem to do much once he shows up.