It's a fun theory, but ultimately isn't true. We know for a fact that the Hero's Shade in Twili Princess is the Hero of Time, who is also the very same Link in Majora's Mask. Now, the Hero's Shade is very clearly an adult version, so that means in all likelihood, this Link managed to become an adult. Given that Twilight and Majora are in the same timeline (HH), Link lived. So, he isn't dead here