You don't want someone getting access to your account two years from now to be able to access every old message. Consider every message a separate object that gets encrypted. The keys are changed/updated each time a new device is added to an account. That new device only knows the new key(s), and thus can only decrypt new messages.
The messages I don't want people getting ahold of are either created with expirations or I manually delete them. I couldn't care less if someone 2 years from now can read a chat log between me and my mom if it means that I can actually read them 2 years and multiple devices away from now as well.
What I don't want is to be forced to message with one app for secure chat and something completely different for daily driving. It's a pain, and nobody in my circle is willing to do it (and I don't blame them).
This whole 'I don't care about history' discourse can change greatly for some people after a loved one dies, for example.
I mean, in a sense, all software is just a vehicle for the data it processes, and the actual value lies in the data - not the software.
That data is chats for Signal. If they think that's worthless, even just the history of it, it means they don't really value their own application.
They are wrong.