This is a clear weakness of airports: where people regroup even before reaching the terminal, with no surveillance or security checks whatsoever. And that's not just in Brussels.
Metros and mass transit are generally the most lucrative target, as we see here, and as we saw in London 10 years ago.
This consisted of a small interview on what I've been doing in Israel, where I am going to, with whom and who packed my stuff. This all was before even entering the actual terminal building.
People interested in killing others will just go for the easiest place, so they will just bomb the line at the scanners entering the terminal.
I talk to a lot of frequent fliers, and almost every one of them has said that there is no point to a terrorist trying to bomb a plane any more. The easiest target for terrorism is simply the insecure portion of an airport. We saw this today in Brussels.