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.
An honest node doesn't help an SPV except when it isn't connected to
any honest node. Otherwise the honest node can do no more than "not participate".
The misunderstanding that full nodes are helping seems to stem from a misunderstanding of the network. Everyone is connected to everyone in a few hops. An attacker needs only
one peer in order for all the others to be pointless. The marginal gain for the network in terms of "sybiling" SPV's with verifying nodes is completely negligible.
There are 1000 mining nodes. As I asked before, why would a full node improve the security? By preventing connections from these mining nodes to light clients? That is not possible.