I want to dissagre since I'm thinking as a pragmatist, the end justifies the means but....

They are crackpots

Well, same, and I'm sure that there are some pragmatists in Daesh, but yeah, there's that big crackpot element which you have to factor in.
Now Al'Qaeda strike me as a more pragmatic organisation and I would expect them to have links to non-Islamic criminal organisations, because they have a wider international support base, but they are more concerned on reasserting their power in Middle Eastern affairs than striking at the west at the moment.
Still, it's possible, I can't rule it out that Daesh or Al'Qaeda would strike at the Olympics, it's certainly a prime target for it, but equally I wouldn't be surprised if they didn't and decided to let all the other myriad problems in Brazil do the dirty work for them ("Hajim, there's no point in destroying the Olympic stadium, it already falls down by itself! We are just doing the infidels work for them."). Now the 2022 World Cup on the other hand....