The difference is that Link is an established character who has been around for more than 30 years.
Yes, but much in the same way that Doctor Who exists. There is no one Doctor, as there is no one Link. If five people are asked to picture Link, they might come back with five different interpretations. Where the Doctor "regenerates", Link is "reincarnated". There is a fluidity to the character where the entity takes shape in many ways, none of which preclude the possibility of a female.
Splatoon, Fire Emblem, and Pokemon feature playable avatars because the games are made to be that way.
Yes that is a conscious choice made, but the games aren't built around gender to the extent the experiences changes. If there wasn't an option it would still be very much the same game. All it really effects is the aesthetic (and a very small part of the marriage system in FE).
You can call Link an avatar character all you want, but since the original Legend of Zelda Link has been personified more and more as a character and has since become something more than just an avatar figure.
In some titles he has gained more personality, but typically that personality, past courage and a sense of adventure, is inconsistent.
I'd say Link has more of a character than even Mario at this point.
Even if true, those aren't exactly high standards.
Plus, if you're saying Link has become more and more personified, you're implying his characteristics have increased over time. His gender hasn't. He's as much a male now as he was in 1986. And the personality doesn't have to change if he's made a female... it probably wouldn't.
The franchise already has a lore to adhere to, and one game in a franchise of more than fifteen installments with a playable female Link clashes pretty hard with the rest of the established franchise.
Really? Does getting a pair of boobs really clash with the lore of being reborn to claim one third of the triforce in times where Hyrule is threatened? How exactly would the lore change if he were female? It's a surface change, it's a change of aesthetics.
I might as well start asking to play as female Mario or male Samus. Gender swapping in video games is fine to me as long as it's not with an established character, especially one that's been around for so long. It feels wrong.
But Mario and Samus are a single individual. The same Mario that's in one Mario game is going to be in every Mario game. Same with Samus. If it were a different Samus in every Metroid game with a different design but the same base role I'd say sure, make one male, why not? But it's not.
"Link" is an entity that is reincarnated into multiple characters who are all named Link. If it was one single Link in every Zelda game, then yeah, it would legitimately be changing the character, but one Link isn't necessarily the same as the other. So what if one's a girl? The character is staying the same, the proportions would just change a bit, which happens ALL THE TIME with Link. It's just an extension of the character, it's not an overhaul.
The only problem I have with voice acting is that, besides possibly not being good, the Zelda franchise typically has had areas in the games with massive plot dropping. To have to sit there and listen through all of it isn't an idea I'm very fond of. I'd rather read through the dialogue at my own pace when it comes to those long sessions of continuous plot. Thinking about what it would be like to listen to Zelda constantly speak to you in the middle of Skyward Sword with all of those story reveals feels like it would get grating.
Yeah, it would get tedious if one had to sit and listen and not be able to do anything during the times of dialogue, but that already sort of exists in Zelda anyway through text. Games that make proficient use of VA don't let it hamper the experience or let it become heavy-handed, they include dialogue during times of gameplay, such as traversal (of which there would be plenty in an open-world Zelda) or through cutscenes. There are many ways to screw up VA, but there are also many examples of VA done right.
It doesn't necessarily have to be the kind that sits you down and doesn't let you skip or escape from the ramblings of plot exposition.