Defaults are set on your phone I think, so unless your phone is compromised (in which case the default setting for disappearing messages really isn't the thing you should be worrying about), it doesn't really provide a signal in large-scale blanket surveillance.
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.
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.
I don't reckon Signal disappearing messages have much to do with "large-scale blanket surveillance of the whole population". This seems more about reducing potentially incriminating evidence in the absence of blanket surveillance.
The problem is that all personal data is potentially incriminating and harmful if not tightly controlled. I am not thinking about actually people/governments trying to do bad. It is a matter of chance and false positives, that plainly increase with the amount of data. There is cases when it is totally legitimate for law enforcement to seize all communication. It happened to me once and it does not feel good if all your communication is seized and searched. You give away personal information of so many individuals. Actually one should in those cases inform everyone about such information 'leak'. Alternatively having a red button that deletes/hides all your data, would also not look all to wise and it will put further stress on you in such a situation, in which you know that most probably everything will turn out well for you. Even if not giving away third party information would be the ethically thing to do , you will not likely do so even if the claims against you are not justifiable. So better delete your conversations and reduce your footprint. This is also good for email. Everyone should just do what we expect from companies...
I've switched most of my group chats with friends to autodelete.
While I haven't had my devices seized, two acquaintances had their device seized for an investigation. They were just on the wrong place at the wrong time.
This meant that months and months of our chats were potentially read by a third party.
But that is exactly what the default should be and what's needed to thwart large-scale blanket surveillance of the whole population.
If the mere fact that ephemeral messages are enabled raises a flag, that kind of defeats (part of) the purpose.