What kind of incentive does the attacker have to just harm a community of a distributed system?
Hostility toward a community in which the target currency is particularly popular.
“Some men just want to watch the world burn.”
Mind you, I say this as a crypto currency outsider.
That's true as long as cryptocurrency itself is seen as fringe; it's less true if cryptocurrency becomes generally accepted.
Of course, a nation-state or other actor interested in preserving the role of fiat and keeping cryptocurrency on the fringes is also a possibility.