Thanks for your anwer.
I am not entirely sure that all the bikes are thru axle.
First it depends what type of bike we are considering, i.e. Fully or Hard Tail. On a hard Tail it is quite easy to recognize that the thru axle stiffen up the structure of the bike. since it generates a definitive strong bond.
Regarding the fork, I bet it is the same, but I can not really share my experience on the topic since I’ve mainly been riding Lefty’s so the thru axle is no question…
Then if we consider a Fully, tough to recognize where the stiffness loss is coming from… Many construciton are using seperates bearings for the all connection in the damping chain. Some brands are also using thru axle technologies for the rotation points in the frame.
A global tone, geometrically speaking, a thru axle will always be stiffer than a STD 9mm QR. Then I’d believe that Thru axle may become more and more STD since more and more XC bikes are being designed so.
Hope I answered your question.