The funny thing about Zelda is that she was basically not just tied with Sheik in the Smash 2 Poll(Ganondorf did beat them out, yet he only got in due to being a clone option), but it's made pretty clear on the website on both pages how important the transformation really is. It's their core concept. So the idea Zelda could get in without said Sheik is... extremely low. That would mean she'd have to have a strict concept of playstyle that doesn't include her unique transformation as well. Yes, they got split later, but that's after they gained more divergence with abilities and options in the series. Sheik didn't need any due to already having a very simplistic moveset as a ninja that at worst was difficult to animate(but easy to imagine). Zelda had to borrow a move from a different version of herself to make her next moveset work. Sheik ironically never borrowed any moves to begin with, as they're all made up or canonical moves she does. It's still only two canonical moves, but still.
To further clarify Zelda's abilities, she did have some kind of magic, and you can barely say that her grab and throws slightly reference her psychic abilities(as she could force open doors, so a form of kinesis), but these are stretches and show how difficult it was to come up with a coherent moveset with her massive lack of abilities. Which means literally her only legitimate move to transfer over at the time was... Transform into Sheik. So besides having the same votes(meaning that to the fans, they're equally popular at the time), she didn't exactly have obvious moves to use either. So of course a concept that makes both work well as a team(I note concept. Obviously the balance was bad, but to be fair, both Melee and Brawl has massive development issues and are highly unbalanced. Ironically Brawl's is worse despite being delayed more. Leave it to Sonic to also ironically slow the game's development down. Whoops?). 4 had its own issues, but it wasn't all that balanced either(Zelda still sucks). Ultimate was the first time Zelda wasn't trash tier. I don't remember how good she is now, but they definitely buffed her. The thing is? She wasn't going to magically be good in Melee either way. She may never have gotten better till Ultimate at best. Mayyyyyyybe in 4, due to having a better team for balance(and 4's balance wasn't as bad as Melee and Brawl. 64 and Ultimate do hold out for the best balanced games thusfar).
Sheik's going nowhere nor did she ever have a chance to beyond Brawl at best. She's also a separate person now, so that factor doesn't even work as an excuse for a cut. Shiek is a different Zelda from the current Zelda, which wasn't even surprising they'd go with after a while. Just like all 3 Links are different people(at least now they are). We've had two unique Links minimum as well otherwise. Like, I get the point with Dr. Mario(though to be fair... it's not like he would've been a costume either way. Definitely not during Melee or Brawl. 4 maybe... assuming Sakurai didn't think the Megavitamins required different properties. That's all it would take to give him a slot. Just remember, said groove system from other games is not plausible in Smash. They screw with results. Whether it's cause Sakurai can't find a way to program it or don't find it's worth the effort when you simply get more for a minor amount of work doesn't much matter though. It's a win win situation for players in general because it gives more diverged options. Literally the only issues we've had otherwise was some Echoes being... very poorly done, which is 2-3 at best, though).
Besides, this whole "two characters who sometimes are the same person can't be playable separately" never made sense in a fighting game. It's nothing but more options to begin with. There is literally no downside to having more options beyond "Wait, I actually have to do more to 100% the game?" which not a lot bother to do cause it's not actually important at all. The worst of it wasn't even Ultimate(only cause of a lack of modes), which has the most characters, but Brawl, where you had to defeat the Target Test variants 35 total times, but 5 times more. 4 is nearly as bad due to the Wii U version's achievements with Target Smash, since that's 30+ characters times 3. Ouch. Hell, as of now, Dr. Mario and Zero Suit Samus are the only ones to be this case(as there is no second Samus specifically. Unlike Zelda, Link, and in one case, Ganondorf, there's never been two separate Samuses). Related, as there is another Ganondorf(and Ganon), they can just use the FSA Ganon(who is not the same as the current Ganondorf in Smash, or actually any mainline Ganondorf who ever directly appeared, in Smash or a physical appearance in said games. For whatever reason, Ganondorf in FSA is merely a mention while only his pig form, Ganon, legitimately appears). Likewise, it's very unclear if the one in HW is actually the same one too, but it'd have to be the OOT variant at best, but remade, or a completely new for that story. At this time, since BOTW has no official placement, that's yet another Ganon who is technically separate(unlike the FSA, who is officially a separate person, BOTW could be considered at the end of any timeline too. Which is odd, cause that makes no sense. But, well, we don't have enough information).
Besides, if you wanted it to be realistic, Zelda would be nothing more than an entrance animation as she transformed into Sheik, the only fightable form Zelda ever had at the time(among the mainline series. Yes, she's fought in a cartoon and in the CD-i games. Except she was literally just a swap of Link for all intents and purposes. She wasn't her own thing. Even though she uses the 3 Goddess Spells that Link did, they're done entirely differently, still at least diverging it). It's more accurate to say Zelda got in because of Sheik and Sheik got in because she's Zelda(even befitting how the website describes it. They both transform into each other is literally the first thing on Zelda's page alone. And they didn't even bother to reveal her first, but Sheik got the E3 bit. It's pretty clear the ninja held importance to some degree). Again, package deal. They complimented each other as a concept. Zelda doesn't get special credit cause she's been around longer either. She would have literally zero canonical moves if it weren't for Transform, which puts a lot of light on how they represented her. ...Which is to say, actually accurate to a much bigger degree by including her alter ego. Also worth noting that they were always noted as separate characters to play as. Again, I'll reiterate that fighting games(if not many rosters for other games) don't care about "same person", because it's completely irrelevant to making a proper roster. What's important is said characters and how they play. Smash isn't even bad about it. It doesn't have 5 or more of the same person either, just in different forms. It's only at most ever had 2 of the same people, and not consistently. Especially when all 6 Zelda characters are overall different people now.
If you want to make it funnier, want to know what happens when you add Midna? Well, she's going to be using Wolf Link, as in TP Link. So that's a fourth Link who is still part of the playable moveset. Welp. Maybe, just maybe, the fact that sometimes they're the same person or based upon the same character is not really... relevant at all when making a roster? The bigger issue, either way, is that said Links do a terrible job of representing more abilities than a few. They're different people, but they barely delve into the constant unique abilities. The closest we've had of divergence is ironically Zelda(mainly thanks to Sheik existing in a way that made her current moveset plausible to create. As they were based around balancing each other out at the time) with the 3 Goddess Spells. Otherwise, we've had mostly the same thing with playstyle differences, while despite this, none of the 3 Links play that similarly. They don't even have the same proportions for that matter(which speaking of Echoes, is a hard requirement that their bodyshapes is barely different at all). That's the reason Young Link nor Toon Link can be legitimate Echoes. They don't work that way.
