If every phone (or most phones) collect all messages, because it's the default, only either phone of the conversing parties needs to be eventually compromised to leak the entire conversation, even years later.

Sure, and if everybody uses WhatsApp and stores all messages on their phones, the entire conversation can eventually be leaked as well. If you don't want conversations to be stored, you can choose not to store them just like you can choose to use Signal. I don't see the problem here.

Your peer will likely still store the conversation if it is opt-out

If you turn on disappearing messages, their Signal client will delete the messages unless they turn it off again. And of course they'll always be able to take a screenshot of your message before it disappears - it does not work if your conversation partner is your adversary. But if you care about messages disappearing and your partner doesn't mind, you can set them to disappear and they will disappear from both your devices.

> Defaults are set on your phone I think

Yes, the messages are only on your phone; Signal doesn't have them.