Underlying Cause of Head Gasket Failures?

I tend to agree with this hypothesis. Just yesterday I heard from a Toyota mechanic who found a loose head bolt and corresponding head gasket leak. Makes you wonder about the thermal cycling argument and why Toyota added the steel and foam spacer on gen4 to keep cylinder temps more consistent. Knocking is clearly a Toyota concern even though some pundits here may discount it.

The structure of the water jacket spacer (Source: Toyota)

In order to achieve optimized cooling by means of the cylinder bore region, a water jacket spacer consisting of stainless steel and foam rubber has been adopted. This involves a foam rubber with a brand name of EXPAD being placed in the coolant passage at the bottom of the cylinder bore. This allows a large amount of coolant to flow over the upper part of the cylinder bore, which becomes very hot, and ameliorates knocking by raising the cooling effect. At the same time, allowing a smaller amount of coolant to flow to the lower half portion of the bore to maintain a high temperature. With these techniques, fuel efficiency is achieved by making the expansion factors of the upper and lower cylinder bore even, and piston action friction is standardized at a low level.