I do understand why some people are a little disappointed that totk Zelda didn't stay a dragon, it would've made for some superb angst.
But like, lore wise I can understand why it had to be done.
What I mean is, well, think about it.
The Triforce is bound to these three specific souls, cursed in a loop of reincarnation.
If one of the souls is suddenly stuck in an immortal form, with their sense of self stuck in a state of sleep-like dormancy... Well, that sorta messes up the whole 'reincarnation' aspect of their prophecy/curse/what-have-you thing, wouldn't you say? (plus it would, like, destroy the very concept of a 'princess Zelda'. like, now that she no longer reincarnates, she'll never show up as a princess ever again down the line. Imagine that in a few centuries, the very concept of 'Zelda' is lost to time and erased from the story. no one remembers the Triforce of wisdom used to reside within a mortal hylian. no one remembers Zelda.)
Plus, Zelda's got the Triforce of Wisdom. Can't really utilise any wisdom if you can't actively think or communicate with the world- outside of occasionally flying down to help the next reincarnation of link fight ganon I guess?