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This sort of failure mode, either catastrophic physical collapse
or loss of key control surfaces does happen and is a significant
contributor to fatal accidents in light aircraft.
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Well Andrewc, you're right it does happen, but you're wrong in stating that it is a significant contributor to fatal accidents.
Structural failure is virtually the least common cause of fatal accidents.
Start with all the pilot caused varieties, continued flight into IFR, landing and takeoff incidents, mechanical (i.e. engine failure, running out of fuel etc, etc.). And then you get to structural failure - so long as you have already captured pilot error over-stressing the airframe due to un-approved aerobatics, loss of control, dis-orientation.
There is no doubt the ballistic parachute gives a great sense of safey - but it will never significantly reduce the fatal accident rate.