When decentralized systems are illegal, only criminals will use them. Anonymously. And very likely, using your devices as botnet slaves. But pretty much, you get what you select for.
The fact that you can't decentralize legislation and physical governance is not insignificant. You can't block the influence of preexisting powerful actors. Those factors do have the power to destroy the decentralization movement, and most likely will.
Maybe those "powerful actors" could prevent decentralization from becoming mainstream, but they can't kill it. Consider marijuana, for example. Use has been demonized for decades by the US and its allies. But that didn't stop an appreciable fraction of the US population from using (or at least, trying). And now it's becoming legal in more and more states.
For Internet decentralization, the driving factors will almost certainly be porn, gambling and prostitution. To the extent that they're driven off the clearnet, demand for them will fuel growth of alternatives. Freedom of expression is essential, of course, but it will be just a side benefit.
The more decentralization is suppressed by "powerful actors", the more it will be dominated by other "powerful actors". That is, by organized crime.