I guess the point is that Haskell is more expressive and declarative, which makes this almost (not totally) the default behaviour of Haskell library developers. And I guess the resulting code would be easier to parse (by a human) because of its declarative nature. It would also be easier to test due to its functional pureness.

Whether that merits dumping the incredible amount of man hours that have gone into OpenSSL is debateable. I'd probably say it's worth fixing OpenSSL and trying to refactor it into a more provably sound system.