Essentially, lets pretend the last Bitcoin is mined TODAY. It costs you $3.25 a day in electricity to run your Antminer S9 and you only make pennies in profit. Mining is impossible.
So what is a miner to do? Shut his rig off and watch the blockchain burn and fade into obscurity as Wojack walks into the fiat bank with his McDonalds hat to open an account?
But if you are a malicious actor, and have a butt ton of money to spend without skipping a meal or losing your yacht, you can buy the power to vote everyone out. That's unfair.
You said yourself, a rich person could just buy up a server farm and attack. This actually happened to Bitcoin Gold which forked to an ASIC resistant PoW algo.
That'd be like if Jeff Bezos bought half of Congress and suddenly gained the ability to pass whatever legislation he wanted because he owns the majority voting power.
That's exactly why we need ASICS actually. If you want to affect the network you gotta buy them and contribute to the hashing power. ASIC resistance wrecks that incentive structure.
Each miner only allocates mere handful of percent of CPU usage to mining. The costs to mine are now minimal. So too are the profits. But the barrier to entry falls massively.
With mining pools actually storing the blockchain, passing work down to the miners, crypto mining can become a background process on every device with little to no footprint.