What more could Syrup bring outside of being a rival and plot accessory to Wario in his games? Their relationship is very strained and not in the classic way that Bowser and Mario's is (big bad and hero). Sure, it may be more creative and an original idea but how does that fit in the formulaic approach to Mario games? Specifically, the same formulaic template that doesn't like to venture out of it's creative box aside from it's RPGs and hardly in it's spin off? The same series that is created by the man who doesn't like to add more depth and pronunciation on story and rarely ventures outside it's routine?
Waluigi a character that objectively, is less creative than Syrup is, still remains in that "Super Mario Bros." formula while not being a complete rehash and having a sense of a charm that is conspicuous to most of his fans.
If you're arguing in favor of her appearing in more Wario titles only, I agree, but when it comes to being paired off with him in spin-offs and replacing Waluigi (far too late for that) I heavily disagree.
Our problem is... well, you pretty much put it into words. The mario roster has become formulaic as fuck. It wasn't like that from the start, however; Nintendo in general took a lot of liberties with all of their games back in the day, there was a lot of experimenting even up to the GameCube titles and those are things you don't see anymore. I personally have a big problem with Waluigi because of that: his introduction cements the formula of the 4 plumbers people need to see in order to make a pattern. And then eventually we have a MKWii sorta roster where everyone has a clone like if that was Ninty's trademark way of doing things.
I just keep thinkin that if Syrup was brought back at the time maybe they would have taken a different direction, since there is no way to make her fit in a pattern but she does blend in nicely enough to make a
theme instead. You get the protagonist, the partner and the antagonist for both the plumbers and the ladies, as opposed to just 'the main ones and their clone'. This added configuration that Waluigi's presence doesn't allow could've also meant that Daisy might have gotten to keep some of her original charm through the years as there was more point of comparison than just 'Luigi's Peach'. I dunno, maybe I'm speculating too much, but you never know how much it would've changed things if the formula hadn't been allowed to consolidate itself in the first place considering it's way too late to change it now.
Hell, if not the general formula, at the very least he reduces Wario to a pathetic evil clone by confirming the 'wah' pattern, completely disregarding everything that came after Super Mario Land 2. At least the farting is still unique to him.
There are many hints to the Waluigi character in his animations, voice acting, and profile and statistics that shows he is at least a slightly unique character from Wario and most of the cast.
If a writer is innovative enough to come up with interesting ideas for Waluigi why would that be a problem?
Well... why haven't they done it, then? What makes Waluigi so irredeemable in our eyes is that every attempt at giving him some sort of personality has resulted in a clusterfuck of traits that end up giving him no identity. If they wanted to keep him after the initial appearance, logic would be that they make a better character out of him, but so far his only consistent theming has been that he's a pathetic villian (an extension of how Wario is treated in the spinoffs) and a partner to a more important character (as a counterpart to Luigi). Any gimmick that would define him has gone from piranha plants to swimming to dancing to being flamboyant and none of these have resulted in anything, nor have the traits attributed to him affected his gameplay in any way (as ChanceTime put it) pushing the idea that the developers have either been struggling to give him some consistency or quite plainly use him for whatever they need at the time because he has nothing that's unique to him. He's an empty shell of a character. And as opposed to Daisy since you brought her up earlier, Daisy has the advantage that she
started out with a theming of her own and lost it along the way due to Ninty's obsession with patterns; Waluigi's never had anything and they never bothered to fix him, but they still keep him around just to fill that space.
Our main concern is why would you bother so much in fixing something that's been a terrible idea from the start instead of opting for replacing it with a much better thought out character? personally the trait I'd stick with for him is dancing since DDR: MM was the only chance they've ever given him to do something without sticking to Wario like a parasite and it resulted in a complete waste. ...But even that thing Jimmy T. does it better.
What makes Waluigi so important that people feel the need to defend him so much? Is it because people are so used to the pattern they can't imagine it gone? Is it because the irony of liking such a bad character is too delicious? Is it because he's so much of a mess it feels like the perfect empty canvas to drop headcanons on? I'm genuinely curious because I don't think funny animations or details from the manuals amount to anything if they result in a mess of a character, and I can't wrap my head around why that's so important to the point of starting every argument with a ''just to clarify, I don't hate Waluigi'' disclaimer.