So many users expect their entire decade+ history of DM contents, attachments included, to be available wherever they are and on any device, gated only by having their login/2fa or passkey. Switching to E2EE would be a major overhaul of that expectation, and it would be a huge task to train users to now keep their encryption key safe, backed up, and available across multiple devices.
Although, mostly unrelated, is that they absolutely are going to have to cull old attachments eventually. There are attachments sitting in their GCP buckets that haven't been accessed since 2015. I'm sure their storage bill is in at least a few million a month at this point, even if most is marked coldline.
That’s not the issue. The issue is that Discord believes they deliver value through aggressively censoring their platform. e2ee prevents that.
e2ee also doesn’t prevent a user from storing their long term keys on the server to be retrieved on new devices and decrypted locally so they can access message history. e2ee does not require PFS.