One day it will be feasible for someone's phone to receive all messages sent by every other person on earth. They can then try to decode every message till they find one that their private key can decode.
You've now made the perfect privacy messaging system - by sending all messages to all users, no quantity of network packet sniffing/timing analysis/evil nodes can figure out who is talking to who.
Also a method to reduce the amount of traffic required was called streams. Instead of reviving all messages, you would be able to know what stream the receiver is on and then the receiver would just listen to all messages on stream 5. This scales up infinitely since you can just balance the network so all streams have hundreds of thousands of users on them.
Would there be a legit motivation to do that? I get that it's a threat, but would it be possible to gain anything from DDOSing that network?
Seems like either a) you come up with a system for banning spam IPs, or b) the spammers improve the anonymity of the rest of the network by creating more noise for the messages to hide in.
And banning spam IPs is a kind of crap solution. Spammers would be running on home network IP spaces mostly so as those addresses shuffle around you end up banning most legitimate users as well.
I have found my self banned from many sites I am only visiting for the first time due to someone else who was on my IP address before.
That works out to 7e11 bytes per day, or 8 Megabytes per second. That is nearly feasible today...
Of course there are scalability issues but they have a plan for that.