You are basically saying that competition is a tragedy of the commons, which is basically nonsense.
A monopolist miner has some incentive to protect the asset in order to preserve his monopoly rents from txn processing.
Competitive miners lack any incentive to protect the asset because they don't earn monopoly rents. None of the client users have sufficient incentives to buy
security because of the free rider problem. However, they should voluntarily purchase an
extremely small amount.
As you can see, monopoly power is the only potential source of strong incentives to protect the asset. Increasing competition decreases security.
Monopoly is not sustainable without restrictions on who is allowed to mine, however. People will enter to capture rents.
Without intervention the free market equilibrium is a) extremely tiny txn fees, b) extremely low difficulty, and c) a large number of atomistic miners.
You better either a) hope that the protocol is changed, b) hope that security isn't necessary after all, or c) wait for a currency project with a sustainable design to come along.