Finger in the air: 6BTC in fees per block for 'very high' fees.

144×6×365 = 315K BTC per year.

The issue for such an attacker is not the nominal cost in USD, at least initially. It's that this amount of continuous buying might end up being good for Bitcoin's price, supporting 'hodl' mindset/strategy instead of undermining it.

Which has its logic. Bitcoin would be proving its usefulness :)

I also totally understand that this logic is *extremely* shaky.

Meanwhile, the actual reason I've never found this scenario worrying is because bureaucracies will never commit to actions like this, actually buying btc in bulk. They are much more likely to try to attack miners, which is why mining centralization has always felt like the biggest danger to be wary of.