I’d completely agree that the stiffness thing is maybe not something everyone will actually notice.
Very simplified, it is more to do with lightweight long forks = bigger bending moment so more stiffness = less deflection = less strain (strain is basically a measurement of localised deflection) = less stress = less fatigue = less snappy steerer = less hospital.
But it all has to be balanced – durable structures have nice gradual stress gradients and the last thing you want is something that suddenly changes in stiffness (or it just breaks at that point).
Stanchions aren’t tapered (forgetting the slider functioning bit) because there are two of them working together (so straight away double stiffness) and already well over 30mm diameter.
The other big problem is the range of riders and usage – I’m a 64kg so could probably ride a 150mm 1 1/8″ fork and never break it. Some riders are twice my weight and huck off jumps with varying levels of finesse…….
Turnerguy – you aren’t comparing the same thing. You need to compare 2 x Magura or 2x RS forks with different steerers (everything else being equal).