The man himself said he wouldn't be in Smash for 3DS/Wii U.
The man himself also said Villager wouldn't be in Brawl.
The man himself can change his mind, what more could we hope for?
All of this!
On top of that, Sakurai is just one man on the team, and thus delegates power a lot to what people working under him suggest. Triple A Video games of this size and magnitude require hundreds of individuals and dozens of programmers and character designers to create characters and a roster.
And if somebody makes a great idea and proof of concept of any particular character, you are damn sure it is going to be considered.
Ridley is an extremely complex character to put together given his body composition, unique skillset and just how different even his movement is from the rest of the cast. It would require very specified programming to add him to toe roster. That is the biggest challenge with adding Ridley ultimately.
If you've seen the popular Project M/Smash Legacy mod of Ridley, you realize how having a character of that scale can be complex to get to work. And that mod of Ridley (with dozens of hours put into it) is far from being up to "Smash quality" (despite it being very impressive).
With that in mind, Smash Switch not being a "2-in-1" development (which is more like "basically make 4 games at once in order to make everything almost the same) like we saw for Smash 4 on the 3DS and Wii U eliminates huge negatives. It eliminates the 3DS' insane limitations (which obviously couldn't even handle him as a boss), it eliminates the Wii U's Gen 7.5-esk technological weakpoints (namely having a 2nd screen on all the time and a weak GPU and CPU), and it opens the doors up to more modern technology.
Brawl was held back by the Wii (which is basically a 50%-100% more powerful GameCube), Smash 4 was held back by the 3DS (which in many ways is as strong as a GameCube and in other ways is weaker than an N64; and yes, this definitely held back the engine possibilities of Smash Wii U, which runs nearly 1:1 with Smash 3DS engine-wise), and the Switch has none of those issues.
This will be the biggest technological jump engine-wise for a Smash game in terms of possibilities. Yes, even bigger than the jump from 64 to Melee. That leaves the door open for hope now.