CapitaineCrash
Smash Champion
I think the case of Little mac is very different than Waluigi or Isaac for Ultimate (I'm talking about those two specifically because they're the most request I think):I agree with mostly everything, though I feel ATs are certainly possible; it seems they were never implied to be good enough for the characters specifically, but Sakurai did hope they'd make people happy, and of course there's his joke with Waluigi; "just because you try hard doesn't mean you'll make it into the battle" but idk. Of course, as we've seen with Little Mac, they can eventually become fighters if ever reconsidered, and I think that could happen through DLC, since they don't seem to care about look-alikes as much as some thought (Bomberman mii costume looks like his AT, and there's the Pyra/Mythra color-swap alts.) and ATs can be disabled depending on the stage (so likely could depend on chosen fighters) as well.
-There's a 6 years gap between Brawl and Smash 4. This gives a lot of time for Sakurai to change his mind on some fighters. With the dlc there's like a year gap, maybe 2 at best so if he didn't want some characters to be playable there's less chance that he will change his mind.
-Punch out got a new game soon after Brawl release (Punch-out wii in 2009). It sold more than 1 million copies (I would say that is fairly succesful for a franchise that had no game for 15 years before that). This probably made Sakurai consider more seriously Mac, as he was more relevant in a way. While Waluigi still has recent appearances through spin-off (like Mario golf coming soon), Isaac got nothing.
In the same way, Isabelle role had changed a lot between 2012-ish (when the roster of Smash 4 was decided) and 2016 (when Ultimate's one was decided). In Smash 4, her only appearance was new leaf, but after that, she was the main character of Happy home designer and one of the most important characters of Pocket camp. She was also made playable in Mario kart 8, and she was basically considered the new mascot of the franchise (taking over the Animal crossing twitter and other social media). So her upgrade from assist to playable was logical. The thing with Waluigi and Isaac is that there really is no reason to reconsider them. If Sakurai didn't want those characters 2 years ago, I'm sorry but I don't see why he would want them now when there's even less spot left in the roster.