They're different mediums with different expectations of ephemerality. Chat history sits somewhere between speaking IRL and sending letters (but to be clear, is not a simple combination; it's its own thing) and nobody burns their letters when they move to a new house.
People do burn their letters for valid reasons (or use more naturally ephemeral media like phone calls, talking IRL, or Signal's disappearing messages) but those reasons are orthogonal to moving house or getting a new phone.
In any case, if people want to save chat history, the appropriate response is to support that requirement rather than to tell users that no, we've decided that they actually do not want to do that.
... particularly given that Signal does have this feature--maybe not as smooth or easy as it should be, but still totally functional--on Android; so it isn't even a consistent argument that "we've decided they actually do not want to do that"!
>nobody burns their letters when they move to a new house
Actually that's usually when I finally make the effort to burn old mail that I can't just throw away. (insurance payment paperwork, credit card bills, etc)
People do burn their letters for valid reasons (or use more naturally ephemeral media like phone calls, talking IRL, or Signal's disappearing messages) but those reasons are orthogonal to moving house or getting a new phone.
In any case, if people want to save chat history, the appropriate response is to support that requirement rather than to tell users that no, we've decided that they actually do not want to do that.