Let's imagine that 2025, and we hit the fourth mining-rewards-halving. It's now unprofitable to mine given the existing numbers of miners, and users refuse to accept higher transaction fees, so 50% of miners exit, leaving half the profits to half the miners.

This means that 50% of the Bitcoin hardware is now on the market, being sold on the cheap. What do you need to compromise the security of the Bitcoin network? Why, 51% of the hardware--