Such an attack is possible as the ASICs are centralised to just 1 or 2 companies now. So they could be stolen/paid off etc. This *could* bring about tens of TH of power for a sudden attack out of nowhere.
I just don't see any other attack happening right now, and even this one is pretty unlikely..
A botnets or Google clusters are also very centralized and they both exists. Asics exists only inside the network so they must be produced first for an attack so they are more unlikely.
Entirely false. I suggest you do a little research into the US government and sha256...