You mention that they won't attack if they think we're prepared. Well, maybe. I think it's an accurate statement to say that a specific attack may be diverted to a secondary or tertiary target based on perceptions that a main force attack on a high value target could be stopped. They won't care if they die as 911 shows, just so long as their mission is accomplished. So they may suddenly divert to another target. We just cannot cover them all and there are too many high value targets to cover all of them.
It is flat out going to happen.
This sort of reminds me of something a buddy once told me when he finished the Infantry Mortar Platoon Officers Course (IMPOC) about how a mortar attack can be used to demoralize enemy forces. You send three rounds towards a reinforced enemy position, one that is dug in deep. Like a bunker. So you set the first fuse for an airburst, which causes no damage and makes them laugh and feel over confident. The next round has an impact detonator and that shakes them up and causes some worry....maybe they get on the radio and tell their buddies or superiors how that last round made the dust settle onto them....and while they're talking the third round lands with the proximity delay fuse and it punches down into the bunker....and KA-BOOM! Static in the air.....Word gets around....
You're right we cannot assume that they won't attack if they think we are prepared. I also don't think we can assume that they won't attack if we've been forewarned.
Take a look at the article: "Terror Attacks: Advance Knowledge Does Not Always Result in Prevention" on my website. It's long so I won't post the entire thing here but basically it takes a look at Nalchik. The Russians had warning of that attack and had even publicized that it had been averted several days before the real attack occured.
Something to think about...