I would like to agree on that, but more is not always better :c having a cast chosen wisely is really important to a game and an aspect many people seem to ignore. Didn't anyone else feel Mario Kart 8 would have been nearly perfect if it wasn't for the fact 1/3 of the roster were babies and powerup clones, for example? it might seem like at least the other 2/3 is good and the rest could be ignored, but in reality stupid choices like that lessen the impact of the more important characters: it's like saying Mario and Baby Rosalina are equally important. (For another example, compare how good it felt to get the 'Superstar' title in the first few Mario Parties and how ridiculous it was when 8 came around and a mook like Blooper could achieve it).
The thing with Waluigi and Syrup here is that there was a character needed to fulfill a particular role, and instead of giving it to a character that suited all the needs they decided to put filler that doesn't add anything to the cast. Syrup worked as a partner to Wario, a contrast to Peach and Daisy and an interesting addition to the villians' side. If you see what Syrup's canonical interactions with Wario are, you can see she can easily replace Waluigi in everything he does with Wario in the sports games' cutscenes and the like. Hell, her colour palette was even changed to something that feels like a spit on the face considering how much it resembles Waluigi's, and she would even suit being the Technique specialized character as well. Adding her along with him could only mean two things: either having two characters with the exact same role, or making Syrup an irrelevant addition that doesn't bother the status quo everyone is so used to, ruining her inclusion completely either way.
That's why I think both can't coexist, at least. There is no character that resembles Syrup, except for a clone that now takes up her spot and that would easily make people think she'd be the ''female Waluigi'' if she were to be added now. Put it this way: if Syrup was there from the start, people would laugh their ass off at the idea of making an evil Luigi clone to replace her.