Without wishing to comment on the actual case, whose details I do not know, nor do I know the slightest thing about Shrikes.
The known spar issues have no proven association whatsoever to do with the increased TOW applied in the Australian STC.
But with my engineers hat on, I have to say this is not the truth, the whole truth and only the truth because such a relationship certainly exists in theory.
The fatigue life of a chunk of Aluminium alloy is a function of its entire stress history from new. That is the integral of load applied times length of time applied.
In addition, as loads increase, so does sensitivity to stress corrosion and cracking at stress raisers caused by allowable manufacturing defects.
The "Hard Time" limits prescribed for the replacement of certain parts after a certain number of hours or cycles is predicated on the expected loading of that part. Change the loading conditions and the part may last a lot shorter time before failure.
This is an inexact science, as the continual stream of SB's and AD's even for Boeing's finest attests.
IF (and its a big if) structural failure was a factor in this terrible accident, then it will not be the first time that a well maintained and professionally flown aircraft has fallen victim to the aircrafts previous history.