Rational or not, people will feel less safe if all their messages can just be easily exported to plaintext.

IMHO the point is that it's not rational. Signal is as vulnerable to the analogue hole as any other messaging platform that displays the messages on a phone screen. There was never any credible way to prevent someone who has received your message from keeping or passing on the information it contained. The idea is as unrealistic as the "disappearing message/photo" applications when confronted with any cheap phone or camera separate to the one showing that message/photo. Ultimately if you don't trust the recipient of your information to treat it as you would wish then your only choice is not to send them the information in the first place.

People aren't rational/perfect and Signal wants to keep them feeling safe? ¯\_(ツ)_/¯

(and IMHO there are edge case scenario where the additional friction in exporting messages provides some protection. Particularly when your threat model involves imperfect actors)

edit: here's an example. Let's say I use 4 week disappearing message with everyone I chat with. That's imperfect of course, but let's say right now only about 5% of the people I chat with are proactively backing up/screenshotting my disappearing messages and the rest let messages expire. If Signal rolled out an "export all messages to plaintext" feature, then suddenly that 5% might become 50%. And now a lot more of my messages which used to disappear, are being preserved.

If everyone I chat with is a perfect 'threat actor' that always backups up every message they ever receive, then there's no difference at all. But most people aren't, so practically there's a big difference because now exporting to plaintext (and bypassing time restrictions) is trivial for the masses.