Except, in PoW, you don't even need to lease/buy/hijack 50%, you need roughly 10% of Bitcoin marketcap value to buy enough hardware to 51% attack Bitcoin. Basically if you could simultaneously hack discus fish and ghash.io, you could 51% attack Bitcoin today, right now.
You are referring to pools being attacked. Miners can easily switch their machines to a different pool in the event the pool was to be hacked, or they could solo mine.
If someone were to lease enough hashpower to launch a 51% attack then your attack would no longer be effective once your lease is up. In order for a 51% attack to be effective an attacker needs to control 51% of the network for extended periods of time otherwise the "attack blocks" can simply be ignored