Finding a place for 9/11/01

Joseph Ellis' article in the NY Times recently is very interesting. One cannot argue with his assessment of the top tier of threats to the survival of the republic. Certainly not much disagreement either, about the perceived abuses of power by certain presidents over time. But there is one sentiment within this article--and please understand, I have great respect for Joseph Ellis (I have read His Excellency and most of Founding Brothers)--expressed in the following quotation:

Sept. 11 does not rise to that level of threat because, while it places
lives and lifestyles at risk, it does not threaten the survival of the American
republic, even though the terrorists would like us to believe so.

What about two September 11 type attacks? What if the next time a large scale attack is carried out, then Wal-Marts are bombed across the republic? Would that create a threat to the republic? What if one of the groups of terrorists does get armed with the much maligned WMD?

Have I simply bought this reasoning hook line and sinker? Is the more reasonable approach to dealing with 9/11 to place the threat somewhere towards the bottom tier? Terrorism certainly doesn't pose a threat that is readily visible. The battle lines are obscure at best. Does that, however, lead us to conclude that the threat is not as real?