taper is defined by bulk modulus change over length.
bulk modulus is essentially specific modulus multiplied by modulus of interia, which is a geometric property affected by diameter and thickness.
In the right length rod with two functional tapers, you can't tell the difference in bulk modulus change, i.e., taper, only the weight difference between the two rods.
Is anyone honestly going to say, I don't like that 8-1/2' rod because it's too light?
if you tackle the same bulk modulus in different MOC, you get there by changing the diameter and thickness. What happens with shorter and lighter line-weight rods in the wrong MOC, you can't make a rod that's durable in the higher modulus MOC without adding thickness, which increases bulk modulus, and makes the rod stiffer and faster.
E-glass is the only material that will make a good progressive rod taper 7' and below (20M modulus). Check all the attempted graphite rods in those lengths - they're tomato stakes - same holds with cane, which is a working equivalent to S-glass (28M modulus - S-glass and cane will make short fast-tip para tapers.)
Though I have to give credit to the 6'9" Fisher graphite para taper, which is a great-casting rod.
7-1/2' is right around the overlap where it's possible to make a decent rod from any of the available MOCs. Certainly e-glass, S-glass, cane all make great 7-1/2' rods.
I have one blended glass/graphite rod in 7-1/2' 5wt, a Lamiglas Perigree, which is a pocket rocket, and a jewel (and almost too light to balance with a reel).
Joe Fisher blended modulus to achieve his great progressive graphite tapers - GT40 is a blend of 38M and 42M modulus.
When you get to 9' rods, and especially larger-than-medium line weights, it's obstinate not to fish graphite.
My Japanese XUL salts rods would make a perfect super-progressive dry fly rod, much like a Thomas Light Special, Heddon T-taper, or under-lined Water Witch.
the core of the rod is linear glass fiber, which creates a finer super-fine tip than even DRS can build.
70% of the rod has a layer of graphite fiber, which speeds up the mid.
The horsepower butt, equivalent of a cane flared butt, is a layer of weave graphite
this makes a rod that will cast 5g, protect 2-lb test, and has the butt to horse a 23" seatrout
In this 7'9" rod, you could accomplish the same thing in e-glass, but would be a 5 oz rod instead of 2 oz.
It would be interesting to see rod designers tackle the perfect 7'9" light-line dry fly rod in mixed MOC