I'm not disagreeing that we should ideally have done a lot more to stop IS sooner, though bombing campaigns won't work and I'm unconvinced that a ground war would be successful in the long run either unless you guys are up for occupying half the middle east for a couple of decades minimum. Islamic extremism won't die because we keep shooting it, any more than any other ideology does, and I don't think we're doing enough to counteract people getting radicalised in the first place and stop the flow of fighters to IS.
As for refugees, I'm pretty sure IS could get people into France regardless of the refugee crisis. I mean, sure, it's always been likely that with the rise of IS they'd manage at some point to get a terror attack off, but I don't think that realistically there's an obvious correlation between terrorism and refugee numbers - if there was, we'd expect IS to be making more attacks further east where refugee numbers are vastly higher for one thing. France has taken comparatively few refugees compared to Greece, Germany, or Sweden, for example. I don't think there's a good argument for us being less humane to refugees as a result of this; not least because the other option is probably cooping refugees up on Aegean islands until they die in filthy and underfunded massed camps, which would be a complete gift to IS recruitment.
Essentially I'm very reluctant to shed so much of the open-ness and freedom that is what I most value about western countries in the name of decreasing the likelihood of terror attacks; I don't think it's an effective strategy and, worse, I feel like it's letting IS get what they want from us.
I do agree with Glaurung though that it's not work deconstructing this too much before we get any reports on how the attack really happened. I mean, we know it was an IS attack, but I don't think we yet have a clear enough idea of who these fighters were, how they were trained and armed, etc, to make detailed conclusions beyond a certain point. And yeah, I appreciate/share the anger, IS are probably the most horrendous group in the world today and it's really frustrating seeing them do things like this and not having any way to do anything about it.