You used the word "sent" earlier and the phrase "on my device" now. But it seems the app isn't transferring the message to you, only presenting it -- like a website. (Does the message exist outside the app?)

Not being pedantic for the sake of it, just pointing out the language you are using is from a position of ownership -- you receiving and owning a message sent to you, and it is then being deleted -- while the app seems to be retaining all rights with the sender, who merely uses the app to present a message to you, and can revoke viewing privileges at any time.

Once it can be viewed offline, the sender shouldn't be able to delete it.

I think this is a valuable feature for users of the product. Some users like this feature. If you don't, just use another product.

Not everything needs to be a philosophical debate.

I don't think the feature is well known. What percent of Viber users could tell you that messages can be deleted without their consent?

I think that's irrelevant. It's part of the product, is an intentional feature that has value, and is not even an uncommon feature (exists on Instagram DMs.