Well, the history of actual 50% attacks on cryptocurrencies in the past few years seems to suggest that effect of a 50% attack on the value of the cryptocurrency is... pretty close to nil, actually. Admittedly, a successful attack on a major cryptocurrency like Bitcoin or Etherium is likely to have far different impact on cryptocurrency society than one on a minor cryptocurrency, but the ability of the community to twist every bad news into "this is good for Bitcoin" should not be underestimated.