The problem is terrorists will probably not do the same thing twice. Like at airports, the security increased so the typical terrorist adapts and finds somewhere else with weak security. Then they attack, the place they attack adapts but so do they. The next terror attack will not be at another concert venue, I doubt. They'll find somewhere else which hasn't adapted yet.
If there's a big crowd, then there needs to be security. We seem to be too reactive, waiting until something happens before we tighten the security. As long as we are still doing that, then a terrorists will always find a way. Of course there's a resourcing problem, but for an event this big there should always be something.