I think a good, even great adaptation is still possible. After all, people used to say LOTR was impossible to adapt, even beyond the effects needed. And no one ever expected Peter Jackson to be the one to do it.Agreed. Ursula lends herself exceptionally well to poor adaptations due to the both heady and subtextual nature of her books. They could never quite nail it.
I was randomly thinking the other day of who could properly adapt The Left Hand of Darkness and I came to the conclusion that it's nobody.
An Earthsea adaptation just wouldn't be an action blockbuster like LOTR. Films are perfectly capable of having subtext and heady narratives. They all just come through the combination of elements like visuals, sound, acting, editing, and minimal dialogue instead of just narrative prose.
Honestly if you could find a composer who could do for Earthsea what Howard Shore did for Middle-Earth, I think you're halfway there.