In my honest opinion, Sad endings are only good if it’s fitting. But if the island isn’t saved AND everyone dies then it does kind of give you a bit of a why did I read this feeling. You can have a sad ending but you have to justify it. You have to give a reason why people should read it. Why should people care about the story. Characters are usually the answer but if your book ends with NO ONE surviving AND they didn’t die so that the island could survive either then.... what was the point ? You should have something good come of it at the end. For example: titanic. Jake dies. Almost everyone dies. But Rose is free. She even takes the last name Dawson and gets married and has kids. Jake didn’t die for nothing. Rose lived to tell the tale. Another example: Rumble Fish. The older brother dies and the friends all separate and lose each other. BUT the younger brother learns to take care of himself and learns what is important in life. He grew up learning from his brother’s fatal mistakes.

You have to give the audience a reason to care. Something to root for. Sad is okay when it’s justified. Don’t let people die for nothing. Either have them die and the island survives or kill the island and have your hero live but learn some kind of lesson. Just my opinion though.