The HumVee was never supposed to be an armored vehicle. It's a damned good replacement for the Jeep.
There was that, but being alert 24/7 isn't possible. Also, consider that we had very high casualties in Vietnam (58,000 fatalities alone), Korea, and WW2 (first mechanized GP transportation in war).
The year and a half that it took us to smash Germany/Austria/Hungary in the First World War had enormous casualties, and armor was very rare.
Our fatalities were higher still in the War Between the States (only armor was on a couple of trains and ironclad warships).
So if you map U.S. fatalities over time versus armor, you'll see a declining fatality trend that coincides with a rising armor trend, overall (i.e. speaking very, very broadly).
But...that does not mean that we go to all armor everywhere.
It does show that it makes sense for GP vehicles to be bulletproof for small arms fire. It probably means that even our light GP vehicles should be designed with IEDs/mines in mind (e.g. V-hulls rather than flat bottoms).
It probably means that our existing APC's should all have slat fencing installed for RPG protection, too.
Of course, that's just one side of the equation: defense. Offense is still the way to win wars.
What we're all really doing is recognizing that our enemies don't want to face our offensive capabilities anymore. Rather than fight us in the air, enemy nations are mostly grounding their warcraft (e.g. Serbia 1999, Bosnia 1994, Iraq 1991, Iraq 2003, etc.). Then we noticed that our enemies didn't even want to use their tanks against our armor. Now we're seeing that even fanatical terrorists prefer to plant mines first to then later pray that we run over them...rather than fight our ground forces.
So really what we are facing are the reactions to our Offensive capabilities.
Nonetheless, we've still got to improve our defense.