I'm iffier on ROB but I think Mr. Game & Watch is kind of THE GUY when it comes to these weirdo characters. He's the original, he's a popular character to play, and presumably he is very easy to develop due to his design simplicity and static animation. I'm gonna assume that's why he's fairly "low priority" in comparison, since he can be turned around in likely a shorter amount of time than the average character. I agree about Sakurai potentially valuing these characters more than your typical fan, although I also think the average fan is pretty protective of G&W and ROB for nostalgia sake.
It's maybe a bit arbitrary, but I remember this pre-Smash 4 whiteboard doodle posted by Sakurai and it always struck me that Mr. Game & Watch was here alongside all these other obvious staples. I feel like he's one of those characters like Falcon or Ness who have kinda just cemented their legacy through sheer persistence throughout the course of the series.
Yeah, I think about this too and personally I value this a whole lot. Granted I also recognize these characters have some of the least amount of content to offer individually, particularly ROB who doesn't even have a stage. So I'm curious if they would end up being treated as more "second class" lower priority universes since they're not quite as cohesive. I hope that's a nonissue though because each of them bring something fresh to the roster and are new worlds all the same. I think the value of the characters individually should trump the secondary stuff.
Unfortunately even in some more generous roster scenarios you can only cut so many supporting characters and third parties before the competition becomes pretty stacked and those cuts become harder to justify. Just another frustrating conundrum in this puzzle that I'm still not sure what to make of.