Back in the good ol' Cold War days, you had a very robust air defense capability in the active duty strangely enough called Air Defense Command (before that it was the Continental Air Defense Command) complete with SAMs (Nike, Nike-Hercules/Nike-Ajax, even some US-based AAA) and interceptors - F-101, F-102, F-106s, Canadian (as part of NORAD before space became a player) with their CF-100s and CF-101s and a hellishly expensive but really amazing for the time radar lines in Canada and the US, off the coasts in picket ships, early AWACS based on the Constellation, and even oil-rig like sea platforms (see Texas Tower 4 for an odd way for USAF dudes to die, RIP).
As the threat from Soviet bombers declined, costs from Vietnam and LBJ's "Great Society" escalated, DoD was used, again, to pay for everything else and the budget was cut. One result of that was the air defense mission going completely Guard by the early 1980s. 1st Air Force became the keeper of the airspace and more and more alert positions were eliminated until by 9/11 there were only a handful with birds still on alert. And that mission was for the "threat" coming from outside the ADIZ, not an internal one.
So, yes, 9/11 could've happened back in the old days. We weren't watching, nor trained and equipped for an internal air attack as occured that September day. I also think that any self-respecting nation should have a means of protecting its sovereignty, to include airspace defense.
Post-9/11, the kneejerk reaction, besides TSA and the like, was a substantial beefing up of the air defense mission with ONE. Lots of $$ spent on tieing in the FAA radars in CONUS to the sector operations centers, lots of $$ spent on upgrading alert facilities and TTPs, etc, etc. And not to mention the money spent on those actually pulling alert. Which, by the way, is a political decision, not really a military one. Congress and the President mandated the CONUS-based operation.
Without going into some of the current measures in use today, I still have yet to see how splashing a B767 over Georgetown is better than one hitting the intended target; the bad guys still achieve their aim (no pun intended). But it's a feel good capability. But point the finger at the politicians, not the units involved. If not for that mission, even more fighters would be gone from the inventory.
AGRs/extended MPAs/and the like are at the behest of the political leadership. Don't blame the guys who choose to fulfill those missions. Blame the ones who decided they are worthwhile.
brick - AGR (ret). DoD air defense liaison to FAA's Air Traffic Control System Command Center, 2002-2008.