If someone does try to DDos HackerNews, the amount of resources required to DDos would not justify the investment,as there's nothing to gain here in terms of monetary benefit.
I have a very rudimentary understanding of a botnet, but I am sure it's not going to be super expensive if you are trying to DDos a site like this. It would be prohibitively expensive if you are trying to do this to something like AWS.
However, the monetary gain from doing this is gonna be zero dollars. The entertainment factor isn't gonna be likely either.
Sure, some people aren't looking for anything logical, but even for them, it might really not be worth it.
If they were to bring it down for a some period of time,there won't be any schaudenfreude for someone of a certain political ideology performing a DDos to make the people of another politicial ideology suffer. This site isn't overwhelmingly conservative or left-wing or libertarian, at least to me. Its kind of amorphous. Its folks just sharing cool websites, apps, new posts,jobs and, blogs etc.