Exactly. So if he's so changeable, do you really think that the champions shirt would be "essential"? Does it even have any special abilities? I think its just a shirt. Notice how in most Zelda promotional art they just have him in his Hero's clothes? In ALTTP's promo art, the clothes are all the Hero's Tunic. But you actually spend most of the game in the other tunics (if you play it normally and get them in order like a normal person who isn't trying to make the game harder). I think the reason we see Link in these clothes is not because they are important. But more because Link has lost them somehow, I'm absolutely positive that Nintendo wants this as one of the mysteries, I doubt they are just going to up and throw out the green tunic. It's a part of the Legend! I also think that they are using it in the promo art singularly to avoid confusion on multiple things. Maybe Link lost his status as the Hero somewhere in the past and the tunic was taken from him as punishment. Who knows? Nobody for sure.

Again, if the green tunic was meant to play a story essential role in the game, we would have seen it by now! The only way the green tunic is ever going to make it into the game is through Amiibo functionality. That's it!
Also, you've clearly never played SS, TP, MC, or WW, where the green tunic (Link's hat in MC) in those games played hugely important role for the sake of story! So those games literally prove that the green tunic played an important role in all the traditional style Zelda games in terms of story. Which is why in most of the promotional art for pretty much all of those games, ALWAYS featured the green tunic. But NOT Breath of the Wild!
For BotW, we, instead, have the Champion's Clothes, instead of the Hero's Clothes! This means that the Hero's Clothes won't be playing a story essential role this time. Instead, the Champion's Clothes will be filling in for the role of the Hero's Clothes this time.
If you would've actually been following along with the lore of the traditional Zelda, you would know very well that the Champion's Clothes IS a replacement for the Hero's Clothes!