I'm not convinced we need to change the power down at all. In fact, this situation shows why a long power down delay is beneficial. It's a disincentive for trading-entities (like exchanges) to make short-term interference is governance.

The exchanges powered up under a certain protocol. They should have known that Justin, a newcomer to the ecosystem, cannot promise a protocol hardfork giving them special treatment or making a major economic chain. They powered up with this knowledge.