We need to remember, even if someone ever even tries it, then everyone else would go on the defensive and would make it even harder to do it, so they would have to slowly gather as much mining equipment as they could without letting anyone know, and you can't just buy it because it would be known, so they would have to build it, and that would require a lot of time as well. I would guess that its not really possible or at least like it is said, viable. It would be near impossible and not worth it at all. It is just way too expensive for a short temporary attack that will be gone in a few days at max.