I disagree with basically everything. Impa is also a ninja, so people would play her for the same reason.
That doesn't matter anymore. They would rather keep the Ninja they know and love. Not be replaced. That's a horrible idea that pisses off fans.
Sheik is also no longer a fan favorite, cause nobody really plays her in Brawl.
This is completely untrue and you know it. Every single gets played. Hell, the least played characters are pure Zelda and Link. Sheik gets more than enough play. I ain't talking about who wins so much as played.
But her Melee tier list placement indeed has nothing to do with it, as she's just as good in that game without it. Zelda also sucks for the same reason, cause they where supposed to balance each other out.
And that's why both are in. They're popular because they're still awesome characters.
Somewhere it has been said that the key to play with Zelda / Sheik was : ranking up damage with Sheik, finish with Zelda. But people kinda forgot that last part as Sheik could also finish. And quite a lot easier. :/
That's why Sheik is more popular than Zelda.
So yeah, Impa would basically be the best thing for Zelda ever.
Clearly in your opinion, and only yours. She would not make Zelda a better character at all. Not anymore than Sheik did.
This is completely false and unfounded. The only way for Zelda to improve is that Sakurai makes her better. Sheik is still a great character and is perfectly fine. Zelda is not. She's the problem, not Sheik, who is actually balanced more than enough and still a popular pick.