We should keep in mind, In case of a 51% attack there is always the remaining 49% who will keep mining regular blocks. That would give time for bitcoiners to launch a counter attack (bitcoiners get into mining who were previously not, switch mining pools etc).

Also it is immensely non profitable for an entity launching the 51% attack whose revenue depends on the protocol they are attacking. So, any rational being driven by motive of profit will not support it. So, the attack can come only from the government.

In a multipolar world getting governments to agree on a single goal is not possible. Also for any country that goes against such a coalition the reward will be hard to resist. (more mining, bitcoin capital moving to the country) along with increased transaction fees due to reduced number of real blocks.

so what should we do? - we should just let the free market economics to play out and thank the attacker for speeding up hyperbitcoinization!