My orignal theory was that there is something fundamentally wrong with motorcycle design and, perhaps, the chopper/cruiser style was developed to overcome the failings of the standard sit up and beg seating position. I've no idea how comfortable or otherwise cruiser style bikes with forward pegs are for long distances.

Giving it more thought and from some of the comments here I think that OE seats do not get any better as time moves on because the manufacturer is always constrained by the need to make the bike fit as many customers as possible although now they often offer a choice of seats and other adaptions to accommodate different sized riders. Bikes that have had the most comfortable seats, in my experience, have seats which are wide at the front. In order to be able to get your feet down these bikes have a relatively low seat height and a relatively small seat to peg spacing. The exception to this was the 1150GS which had a relatively wide (and high) seat but was very uncomfortable after 50 miles. The 1150 also had a larger screen which should have provided good wind protection which I think is the other important factor for a bike to be comfortable for long distances. I think the actual riding position has some affect as this determines which part of your backside is bearing your weight but I would have thought the seat could be designed to suit the bikes riding position.

It's hard to draw any firm conclusions but if anyone has a Moto Guzzi Spada for sale I may be interested!