I am agree with your thesis, but not entirely with your reasons.
Here's my take: Waluigi shouldn't be added as a playable because he's never earned that right. In my opinion, almost every character present has, in some way, earned the right to fight alongside the other characters, namely: the original eight. The invitation to Smash is like being invited to the video game cool kids club. Outside of Piranha Plant and maybe Corrin and Richter, I feel like each character meets this standard.
As for Waluigi, he's literally the definition of a supporting actor. Heck, sometimes, he's more like an extra. Sure, he's got a big following and I always pick him in Mario Kart, but what does he represent? Literally nothing. He's like an alternate costume given its own slot on the CSS. Virtually every game he's in, he's very much a side character. Mario, Luigi, Bowser, Peach, Yoshi, Wario, DK...all of these characters can exist on their own, and they do a good job at it. Waluigi? Well...there's just no substance to his existence. Until recently, he had as much depth as Toadette, but then she got her own game with Captain Toad, so...yeah. (This is also why even though it was free DLC incentive, I still think Piranha Plant was the worst addition to Smash ever. I recently saw people suggesting Sandbag should have been that character instead. I agree with this 3000%.)
Now you could argue this is mainly Nintendo's fault because they haven't given him that opportunity. That's fair. But until they do, I think he is great staying as an AT. He's still at the club, he's just one of the chefs or something.