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
Except ghash.io owns most of the hardware mining on the pool thru cloud mining, and discus fish is soon to offer cloud mining too.