how exactly does this happen. This has to involve protocol change or is there something in bitcoin protocol that forks around a chain when a double spend is detected. And how exactly is a double spend detected ?
Diuble spend is detected when a longer chain is published to contest the currently accepted chain with orphan depth of more than 5-6. This is huge deal and will decimate butcoin valuation for sure, but just as with acceptance of segwit and hardforks around protocol bugs in the past - there can be agreement made to deploy a version of bitcoin software to hardcode which chain is valid.
Not saying this is easy but when the project is attacked like that, there will be some action.
Not saying this is easy but when the project is attacked like that, there will be some action.