When taking the Eurostar, I noticed the new model has seats that are harder than the old one, which is significantly less comfortable. I couldn't notice if the same happened to airplane seats (it's harder to compare old and new seats in the same way, due to greater variety), but I wonder if there are reasons other than, say, cheaper cost. Is it better for maintenance? Do studies indicate that harder seats lead to people going to the food car more often? Safety issues? Could they be better for blood circulation?
A number of new UK trains have thinner padding on new seats in order to increase legroom - people are taller now on average and need more legroom, but the trains aren't getting any bigger, so they can get two or three centimetres by making the padding on the seats thinner.
I don't know if this is Eurostar's reason, but it is the official explanation for the thinner padding on the new Thameslink trains ("class 700") compared to the old ones ("class 319").