my (limited) experience is the opposite. I have more back pain after a ride with my upright, dutch-style ride, than on my sporty trekking bike, that has a more inclined position and I ride comfortably for longer and on more irregular terrain. Maybe the vertical position is worse to absorb road vibrations?
Could be. My point of reference is The Netherlands, where we have good cycling infrastructure and the roads are typically very smooth.
On a trekking bike your weight is more distributed, since your torso is sort of laying down.
In my case, a change of saddle made a great difference. The two saddles weren't particularly different in price or overall look, but one happened to fit my body much better than the other. Luck.