Encryption is also pretty much useless if the thing performing the decryption is served by the people encryption is protecting you against. It would be trivial for a Lemmy instance to serve you backdoored JavaScript as the decryption Algo.

E2e encryption is a legal device used by companies to remove their liability to give your messages to the government. Because united states v Apple (which didn’t have a ruling but the case made was pretty strong), code is speech and the US gov cannot compel you to change your code, therefore companies that have e2e encrypted comms can say “we don’t know, we have a way of knowing but you can’t make us” whenever the US gov comes in with a subpoena.

Bear in mind this is only US jurisprudence, there exist jurisdictions in which the government could legally compel an instance admin to serve you backdoored JavaScript and read your messages.