All a full node is "in charge" of is which blocks it accepts. All it can do is reject a block and not relay it. But the network is extremely well connected. Blocks will find their way anyway.
The blocks HAS to go through a rogue node which has modified rules. Imagine if there are only 1000 nodes, one can easily setup 200 nodes and use modified rules on them. The higher the number of rogue nodes, the higher chance an SPV client would connect to it and be disconnected from the actual network.
The higher the number of honest nodes, the harder it will be and the network will be more decentralised as a result.