Even if we are to take this seriously if they could brute force private keys, how long would it take before someone watching it reports coins are moving that they didn't sign? How much would it cost do perform an attack, so do you go for as many addresses or 1 big one IE satoshis?

If it's purely to take down the network, it remains to be seen how much damage they can do and how long they can sustain it for

Worst case, theres always snapshots that can be taken for a hard fork