So, please tell me, who can buy 15% of the hash rate, in what time schedule? It's not like, I go in the store and say: "Here is 1 billion dollar, one 15% of Bitcoin hash rate, please".
Still, nothing about my other counter arguments.
if i was a gov or a central bank i'd probably get all the hash i wanted. If nobody would deliver to me i would have the recouces to build my own asics. Does this even need to be explained?
other arguments are false very likely. The claim 50% drop would cause 20 minutes blocks is likely wrong. I'd think they would take much longer.
Let's ask the other way: at what hashrate loss would a block take 3,10 or 12 hours to solve?