The more laterally curved the saddle, the greater is the pressure trying to separate the sit bones. If the carcase of the saddle is stiff, then padding it will not undo the forces trying to separate the sit bones. With break-in, a leather saddle eventually forms pockets for the sit bones to rest in. This provides a fairly neutral slope to the surface supporting the sit bones. It may also help to increase the area of contact around the sit bones without the use of artificial padding.
Moreover, the tension-suspended leather provides vital and local suspension for the sit bones by flexing under road shocks. The human bod has a large mass which cannot possibly accelerate fast enough to minimise road shocks. The rider's lower body remains almost static unless the rider lifts off the saddle on the rough stuff. So only the saddle itself can protect the seated rider's sit bones from a constant and very localised battering.
A suspension seat post cannot react fast enough to protect the sit bones. The saddle itself becomes part of the unsprung weight and can only react downwards through the entire contact area of the saddle. Still leaving the sit bones exposed to road shocks passed straight through the saddle surface. A telescopic suspension arrangement of the normally backward-leaning seat post is not even in line with the vertical forces involved in road shocks. Which usually try to lift the entire machine (and rider) vertically as the wheel rides over road bumps.
One could liken the sit bones to stiletto heels. Both have a very small area of contact with the surface on which they rest. So local pressures are incredibly high. Anyone who lived through past fashions will know the incredible damage which stiletto heels did to floors in public buildings. Several tons per square inch, on every footfall, exceeded the damage you could manage with a good hammer blow!
The same applies to saddles and sit bones. The tiny areas of skin and fleshy padding (if any) between the sit bones and the saddle become the torture victims. If the standard rails provided any suspension (at all) then every saddle would offer equal suspension. This is patently untrue despite the very close standardisation of most saddle rails. Only the amount of soft padding can alter the comfort level on plastic or carbon fibre based saddles. Real-world practice suggests that padding can easily hinder comfort over any distance. It increases friction and heat build-up in poorly ventilated areas.
Logic suggests that plastic saddles should try to emulate the properties of successfully broken-in, leather examples. But without the usual weight penalties. The surface must be almost flat to provide local pockets for the sit bones without excessive padding. The surface must also be locally flexible under road shocks. A gently curved surface helps to maintain the rider's position more securely than a perfectly flat one. The same effect is probably achieved with sit bone pockets. A very slippery surface is undesirable. As is one offering too much drag to the shorts of the rider. Sit bones depressing deep padding may become trapped by the friction of the cover. Which is greatly increased by the great depth of padding resisting any lateral movement.
Many leather saddles start off too shiny, very hard and only gently curved at the contact points. Breaking in causes a local softening of the leather from repeated contact with the long-suffering sit bones. The softening provides the padding, reduces the angle of contact with the sit bones and provides suspension.
The plastic based saddle cannot really improve beyond the condition in which it left the factory. Despite the claims of memory foam the chances are that the padding will slowly degrade and the covering will disintegrate. The sit bones end up resting on the almost naked plastic carcase lying beneath the padding.
Only a naked plastic carcase, like the original Unica Nitor, will probably survive for as long as a leather saddle. Though at the price of being very uncomfortable on longer rides. I know this because a black, Unica Nitor 'Road' model was my first serious "racing" saddle back in the 1960s. Had they made it of much floppier plastic (or even rubber) and shaped it like a properly broken in leather saddle it might well have offered far greater comfort and popularity. Though probably with a much shorter life. It would also need lots of ventilation holes.
As a final note: I have mentioned how incredibly comfortable I find the B17 'Select'. By adding the official Brooks waterproof cover the comfort offered is greatly diminished. A much thinner and smoother nylon cover is much better. Though it is still not remotely as comfortable as the bare leather.
More thoughts on Brooks saddles: Not all Brooks saddles are equal. Certainly not on width. I have suffered for thousands of miles on a B17 Narrow and then for tens of thousands more on a NOS 'Professional'. Finally I bought a B17 out of desperation. It felt like an expensive risk at the time if it didn't work. I bought a pre-aged 'Select' in fact. To avoid the usual breaking in time. As a high mileage rider I was already sore enough not to to have to go through that all over again!
So don't lump all Brooks saddles together until you find your own perfect, leather saddle width. It took me tens of thousands of miles and literally years of cycling to finally relent and buy the wider B17. Now I don't even notice I'm sitting on a saddle. Blessed relief can be yours. Just don't imagine that just any leather saddle will do. It must fit you just as a precisely as a leather shoe must fit you for any expectation of comfort.
If I hadn't bought the B17 'Select' I would probably still be forlornly trying to find a comfortable plastic wedge. I was almost drawn in by the plastic saddle bullshit. The bike shop's measuring device said I needed an even narrower saddle than those I already found horribly uncomfortable. So much for pseudo science, commercialism of overpriced plastic torture implements and cycling! Even Tupperware doesn't charge that much for a simple plastic moulding! Ikea could do proper plastic saddles for small change in a range of tasteful cover materials. It hardly matters what covering is used. A plastic base can neither breathe nor give by a micron.
Like me, you probably thought that a wider saddle would make you look like you had a horribly dated 70s leather sofa stuck on top of your saddle pin. You probably still clung to the illusion of some sporting pretensions as you plodded along at your own, unique pace. So you suffered the slings and arrows of outrageous saddle misfortune out of some false pride. Why suffer when an old cure is at hand? Try a wider, Brooks leather saddle. If it doesn't suit you can almost guarantee to recoup your losses on eBay. So what have you to lose?
The Nidd might as well be a plastic saddle because of its reinforced plastic undergarment. It breaks the age-old saddle rule of allowing a natural, organic material to reluctantly respond to stubborn, oft-repeated, human contact. It might actually respond to having the glass fibre (?) patch ground away with a Dremel or a similar tool. The very thick leather could then be modified with moisture and localised pressure to match the owners's unchangeable anatomy in quick time. Perhaps there is a solvent which will remove or soften the reinforcing patch but leave the leather unharmed? It may be that the patch only reduces stretch but has little real impact on the leather's stiffness. How can one tell? Without an sample to test, which has no applied reinforcing, it is impossible to know.
The Nidd is a complete unknown. I hope soaking it will make it malleable to modification by pressure in the seating area. At the moment it is far too hard to be usable despite having had some 200 miles of riding put into it before it became too painful to continue. Both have had their tension nuts slacked off to the minimum before taking their bath. Even if the water bath therapy fails neither saddle was usable in its present form. Neither saddle owes me so much that they can't become experimental subjects.