USA government debt ceiling

[Georgi Guninski]

On Tue, Oct 20, 2015 at 07:32:46PM -0600, Mirimir wrote:
> 2015-10-20: https://www.bitcoincharts.com/charts/volumepie/


I have very vague idea how bitcoin works.

Isn't dishonest actor with enough resources danger to the integrity of
bitcoins?

According to this:
https://www.reddit.com/r/IAmA/comments/1nisdy/were_glenn_greenwald_and_janine_gibson_of_the/ccjh2mq

---
Nope. The way bitcoin is run requires 51% of "agreement" before you can
break the system. It would cost well into the hundreds of millions of
dollars to get that now, so the system is pretty safe.
...
Minor correction, apparently it would be about $500 million to execute a
51% attack - which is pretty huge.
---

The above was written two years ago, so the number is likely larger now.

Later was claimed that such 51% attack will be rolled back.

How is stuff rolled back in decentralized system of which >=51% is
dishonest?