So what does a message actually cost in terms of dollars, power, time? How does it prevent low volume spam?
So not only does this not help with abusive behavior, it states that abuse will live forever and be impossible to block or filter.
/me shudders
You have the right to say anything you want (though not without consequences). Conversely, I should have the right to literally not see/hear it once I've decided it is causing me harm.
Claiming that your platform/protocol makes this impossible just sounds naive at best and nefarious at worst to me.