Network security comes from many things. e.g. full node operators are more powerful than miners in order to enforce network security. Full nodes are very cheap to operate. I run one on my Raspberry Pi at home. It costs me around ~1-2 USD per month in electricity.

Miners just make it hard to edit an old transaction. To edit an old transaction a dishonest miner has to re-narrate the history of the transactions, for the last ~30-60 minutes. But that means the dishonest miner has to recreate the previous 3 to 6 blocks.

If it ever becomes profitable to mine using pencil and paper, then only pencil and paper miners will have the ability to generate 3 to 6 blocks in say 30 minutes. But since many people will be using pencil and paper it is unlikely one of them will be able to renarrate the history of the chain for the last 30 minutes. It is likely someone else will have taken more aderal to do it faster than you.

You might ask what about the abacus user, isn't that person faster than a pencil paper miner. Sure, but we already established it is so easy to mine, that there are no abacus users left, only pencil paper miners are left. As soon as abacus users enter the mining business the difficulty changes, and pencil and paper miners can no longer find any blocks.