I realize there is no easy answer to the question of "how much security is too much". However it seems ludicrous to suggest that enhanced security measures have not made us safer. A little Google search turned up nearly 100 terrorist plots in the US since 9/11 that have been foiled by authorities. I suspect there are many more which we have never heard about, perhaps derailed in planning stages due to our intelligence networks.
Other plots are certainly hampered by visible security presence. Any location where tens-of-thousands of people congregate in a relatively small area (stadium, theme park) is going to be an attractive target for terrorists. But the presence of metal detectors, cameras, armed security personnel, etc. is most certainly going to help prevent an attack from occurring.
The dozens of unsuccessful attacks weren't foiled by incompetent terrorists or dumb luck...they were stopped by people doing their jobs.
As I write this, it appears possible that screening of social media posts may have prevented 14 people from losing their lives in the San Bernadino shooting. Not worth it?
Again, the "how much is too much" debate will likely continue for years to come. Personally, I've never encountered a security screening that I felt was unjustified and overly intrusive. YMMV..