The overall security does NOT get lowered by running a pruned node.
The 'security' depends on the amount of miner/hashrate. Not on the amount of nodes.
This applies to anyone who wants to run a node. Doesn't matter whether pruned or not.
EACH node needs to download the blockchain from other peers when setting up.
They are NOT damaging the network. They do NOT contribute to the overall health of the network, but definitely do NOT damage the network.
By the way, non-pruned nodes which do not allow incoming connections do also NOT contribute to the overall health of the network since they do not distribute old blocks to new peers.