Despots would add sinks that suck in all messages, by pretending to be a known plankton/connection.
Also assume that traditional isps would first try to subvert it and then try to replace it by providing a similar walled in service - warning from the dangerous open source network.
The whole thing would have to be constructed in away like the TOR-Network, preventing the identification of sender and reciver before Arrival.
Finally i think the problem remains, that any sufficiently enough supressing regime, would force its citizens to uninstall such software.
Not force to uninstall - rather regulate, requiring licensure for operation; giving people a reason to be complacent. (Likely in the name of counterterrorism/child safety/etc)
Still, you have a decent grasp of the situation, which is much appreciated. Anyone who still thinks interpolitical issues can be swept away by some "just invent X" has either not lived long enough to know better or are doomed to the delusion that "it's the next one" until they die.
People will not mind requiring a license for "thing X" (esp. when airwaves are involved), and will not run software that can get them thrown in jail.
Sociopolitical problems do not have a naiive tech solution. [0] People do not cease being people if you retreat further into technology.
Here in the states, the NSA was allowed to view data on telephone calls 3 or 4 levels out from a terrorist. So, for instance, if a terrorist called A, and A called B, and B called C, and so on? The NSA could track C or D.
The argument against that is that it would cover something like 80 or 90% of the population.
Well, there you go. Assume one trillion connected devices and each device is accessible through a "path" of connected devices over 4 or 5 hops, with a percentage_success and average_network_lag time tracked for each path.
Add in a few GB of buffering and the requirement that every connected device use up 10-20% of its bandwidth on public, anonymous access, and you've got a mesh system that's decentralized.
The only other limit I'd add is probably some kind of rules to prevent DDOSing, say a 64Kb message size and a few other things like that.
It's just a math problem.