Believe whatever you like, but Sakurai has never stated this. Don't pass off baseless claims as fact.
If Sakurai really wanted to, I'm sure he could make the decision to promote an assist. I just don't really see him wanting to do that, and I'm sure Nintendo is more likely to believe that new content will sell better. So yeah, in my mind AT upgrades are not so much an "impossibility" so much as a "why though?" And I think it's strange you single out Shadow, Lyn, and Waluigi as I don't think these characters are particularly above any of the other assists. If anything, I think Zero has the best chance of the whole lot of being promoted, simply because he's both the clear 2nd character from Megaman, and he has his MMZ look to fall back to that is easily distinguishable from the existing AT.
I get your view point, I still got to counter argument some things
-"Why though", It's because Alot of the Assist characters are very popular. Waluigi is the biggest example as even Reggie acknowledged it and talked to Sakurai about it. Now whether they can get in over other characters that bring in new content is up to your inpertation, and what the result of the last 2 fighters are.
I will say that there's alot of factors at play though, like whether Nintendo could negotiate with the 3rd parties in time (if they are 3rd party), or other cases like how the Characters with new content compare with the Assists in Popularity, relevance, implementation of potential moveset, etc
-I singled out Waluigi, Lyn, and Shadow because they are veteran Assist trophies, and part of the thing I was saying is that it be weird if their Assists were taken out considering they are very popular characters, and would be downgrade in Role. Characters like Zero and Knuckles were new assists introduced into Ultimate, so it would not be weird if they were not Assists in the game
-I Like Zero, but I'm gonna have to disagree with him having the best chance. For one it's implied he wasn't chosen anyway since his Mii Costume did return in Pack 3 of FP1, and that was the pack when we found out Extra DLC was confirmed.
Even without that, Mario and Sonic are bigger franchises than Mega Man, and I'd say overall there have been more requests for Waluigi and Shadow than there were for Zero, though Zero is still popular request also though.
I will say an advantage is that Zero did have his own Sub-series of Games. Shadow did have his own game (that's really controversial), but Waluigi did not have his own game or franchise, or can't even get into Mainline game.
I will say Zero is more important to the Mega Man Franchise than maybe Shadow, and Especially Waluigi, but Waluigi and Shadow I would say are bigger characters overall, and Shadow himself is still really important to the franchise anyway