here the thing, if they were to have enough incentivised people to buy 2,500,000 asics at $6.3k each ($16b)
.. then there is no point doing a 51% attack if the only aim is market manipulation... they can just market manipulate using the $16b without wasting money on hardware
Why would you go for the most expensive hashrate on the market?
Bitmain sells plenty of gear way cheaper at $20/ths or even s19 at $11/ths, which would bring the numbers down to 6 billion, and this not even counting that you can buy used gear even cheaper, huge batches of gear sold at $4 per this, and you only need to run them for a few days.
So that 16b is closer to a number that would guarantee you more the hashrate for the next few months, at maximum efficiency and not counting how many miners will go bankrupt when their income is halved because you add that much hashrate.
A far more down-to-earth number would be a quarter of that at max.
you are just looking at the hardware cost, not realising that there is an electric cost involved(something that for yours you have 'guesstimated' wrongly many times or ignored in your estimations)
as for saying only needs to run for a few days shows you have not done the math or run scenarios, much like the OP has not