The solution is flooding. Every time you send a DM, also DM 20 random strangers. Signal will drown in the noise.
Discussion
Hmm not a bad idea, kind of like a coin join but for messages
Exactly. Send the actual payload, then junk to other people. To facilitate, the junk could even be flagged (in the encrypted payload) as such so the recipients’ client software knows to ignore it.
Also fine to send junk to sock puppets. Have the sock puppets send junk back, and amongst themselves, etc.
The reason the metadata attack works right now is because all messages are signal. Drowning that signal in noise would not be difficult, in principle.