I don't want to be handcuffed for my own security, thanks. An export facility has little bearing on the integrity of my communications since a counterparty could simply have their phone taken over if they forgot to set a PIN or gave the PIN up to law enforcement. A warning that the exported data can no longer be considered secure (just like when I save a file or photo) is sufficient.

Meanwhile if my counterparty and I have communications in Signal that we want to preserve at scale, it's impossible to do so. A simple example would be that I have years' worth of conversations with my wife that I can't easily back up any where. We could export every picture and video by hand and screenshot or copy the text of every message, but that would be extremely time-consuming and tedious.

That's the basic problem. The data is already only semi-secure in that it's subject to exfiltration without consent. It's just inconvenient. And where parties do consent, it's very inconvenient because it's many many hours of work; in the end, it's just more security theater.