The sheet of plastic forming the inner window in a passenger jet fuselage is pretty flimsy but easily replaceable.  The outer window is much thicker and is typically a polycarbonate material that is pretty tough stuff.

We all remember the scene from the movie "Goldfinger" but in reality passenger aircraft are normally just controlled sieves; there's always a perceptible amount of leakage.  And catastrophic or explosive decompression isn't a certain death sentence in many cases.  Our own Aloha Airlines Flight 243 is a good example.  The incident took place at 24,000 feet and if you've seen pictures of the Boeing 737 after it safely landed at Kahului Airport, it doesn't get much worse than that.  The aircraft had to be junked afterwards, but it remained flyable enough to land that one critical last time.

I agree there will never be another 9/11 aircraft hijacking.  No longer will able passengers sit meekly in their seats and await a resolution or rescue.  They will fend for themselves.