Sakurai having talent and experience doesn't mean that implementation issues magically disappear for him, though. I think it's entirely possible that he might not consider the problem to be worth the effort of solving, or that he hasn't come up with any solutions yet that meet his quality standards.
Having said that, I also believe that Sakurai's spent a lot of time thinking about a new Metroid rep. Ridley and Meta Ridley both appeared as Subspace Emissary bosses. Ridley appeared as a stage boss. Dark Samus appeared as an assist trophy. Each one of these have multiple attacks, which I suspect might have at been drawn from ideas Sakurai might have had for their moves as playable fighters. Each Ridley boss has over half a dozen moves, while Dark Samus has three. A playable character has around thirty moves, so I think you've got around a playable character's worth of animations between the lot of them, at least.
Even if I'm wrong about their moves being leftover ideas from considered playable implementations, Ridley's got a pretty unusual presence in Smash already. Half of the bosses in Brawl were Smash originals; very few series had a boss. Yet Metroid was the only series to get two bosses, both of which were forms of Ridley. If Meta Ridley reused assets from Ridley maybe that would explain it, but I don't even think they shared attacks between them, at the least. Mario and Pokemon had only had one boss each, Zelda and Donkey Kong had none. He's also the only boss introduced in Subspace Emissary to reappear in 4... except he doesn't really reappear, because they just made him from scratch again.
I don't have any inside information so I can't say whether he's playable or not, but I think there has been noticable effort to include Ridley in a big way in both Brawl and 4. I feel that it's likely that a similar effort with exist in Smash Switch. Even if they start with Smash 4 as a base, I don't think they're going to be satisfied with just porting over his Pyrosphere appearance and calling it a day.
