IMO, the 9-11-2001 attack was due to a complete failure of intelligence after 12 years of stupid Presidents.
I agree there was intelligence failure. However, if the perpetrators had been deported when they ceased to be in compliance with their visas they would not have been able to carry out the attack. The circumstances under which they received the visas were also questionable. I subscribe to the belief that it is better to err in keeping a good applicant out rather than allowing a bad one in.