But I believe that a permissioned and decentralized network is also possible. For a node to enter the network, a 51% approval of validators already approved in the network is required.
Who acts as validators? If all node acts as validators, does that means each node need to verify existence of all other nodes?
Humans are very good at identifying bots.
How would human check whether a node is operated by human or bot? Node only need to send networking related data such as User Agent and available service.