And if you try to tackle centralization directly (like banning routers or something), you will often create an anti-centralization regulator, which is, you guessed it, another form of centralization.
So your decentralized P2P network is either small and works good, medium and works not so good, or large and not actually decentralized.
The best P2P networks know their limits and don't try to scale infinitely. For human-oriented network, Dunbar's Number (N=~150) is a decent rule of thumb; any P2P network larger than that almost certainly has some form of centralization (like trusted bootstrapping/coordination server addresses that are hard-coded in every client install, etc.)
