A 51% attack is not just cheap, it's FREE when you have a money printer in your basement. This is the point that is hard for people to wrap their heads around. I find this ironic given the philosophical revolutionary hard money stance many bitcoiners have, yet they can't really comprehend that the Fed can buy literally anything it wants, including hash power.

To put it another way, consider that the Fed prints money to rig the entire treasury bond market and fix interest rates. Buying up 80% of the mining companies and ASICs in the world is not even a rounding error in comparison.

A 51% is not only useful for double spending, it can also be used for un-spending. The Fed would use their hash power advantage to roll back transactions they don't like (this would be any transaction that didn't originate in their own crypto exchange). This puts all the non-Fed miners out of business.

The icing on the cake is that the Fed can use its 51% attack to force Americans owning BTC to sell to them (as they won't be able to sell a bitcoin to anyone else at any price). This ensures that all that money they print goes right back into the US economy, it's just effectively an asset swap like quantitative easing. They can easily sell this to congress for its economic stimulus value.

Miners would not be "colluding", they would be given the choice to sell their business to the Fed and walk away with a nice bundle of money printer fiat, or try to keep mining at a loss until the Fed runs out of money. The Fed cannot run out of money because they just print it out of thin air. Any rational miner would sell their hash power to the Fed. Irrational miners will simply run out of capital and go bankrupt, and the Fed would then buy up their assets and have their hash power anyway.