> PGP is old and broken and messages can potentially be decrypted without access to private key
This would be a lot more effective / helpful / convincing with a reference.
> Because email's default is plain-text, in real world conversations you end up with people replying in plain text with your whole message being quoted
This is more about the UI of mixing "normal" email with encrypted email. If you can insist that everyone install Signal, you can insist that everyone install Enigmail or some other piece of software which refuses to reply to encrypted mail without also encrypting.
> Archived messages will leak.
The specific argument from OP was "Searchable archives are too useful to sacrifice".
I have a hard time expressing how obviously self-contradictory this line of argument is. Insofar as people are unwilling to give up archives, they will be unwilling to give up email. Insofar as people are willing to have email deletion policies, this isn't an argument to get rid of email.
He's advanced the premise that people will never give up archivable messaging. So who exactly is he expecting to influence with this diatribe? Anyone who is unwilling to give up archiveable messages is going to be unwilling to give up email, period. At which point it's just pointless ranting.
If you believe that people want encrypted, archivable messaging, then you need to give them the best solution possible, not just tell them not to want it.
> Private keys eventually leak, therefore it is important that when it happens, the system makes it hard to decrypt old messages (by combining it with ephemeral keys).
So one of the arguments against using PGP is that it gives people a false sense of security; and that having "experts" using it will encourage people to send messages which "should not be sent at all."
In the case of ephemeral keys, you're still trusting the other party to delete both the keys and the messages after the stipulated time. But what reason do you have to believe that the other party is actually doing that? It would be dead easy for someone to write a client which didn't delete the message or the ephemeral keys -- either on purpose (because having an archive is convenient) or by accident.
Wouldn't it be better to just tell people, "You can never guarantee that anything you send won't one day be decrypted, no matter what method you use. Take that risk factor into account when sending any message"?