Good quality steel is quite light as well, about 2.5 kg for the frame and the fork. Add to that beefy 650b or 700c wheels with 42..50 mm wide rubber and you've got a bike which one can confortably ride on most roads, paved or otherwise. The new gravel bike trend is quite in line with this. As long as it's not an UCI approved discipline and the manufacturers start making only race bikes again, one can probably find a decent endurance or adventure model with more relaxed geometry and actual utility. I like the current Breezer Bikes lineup a lot, especially the Doppler. They're made of regular 4130 chromoly, not Reynolds 725 or other heat treated steel types, but they're affordable, very durable and also carbon fork compatible. Joe Breeze is also very experienced designer and builder, being one of the original mountain bike pioneers. If you want a more nimble bike look for a Ritchey or the more premium QBS brands like Salsa or All City.
Dutch style bikes are okay in flat countries like the Netherlands and Denmark, but I'd get a lighter trekking bike regardless.
That said, Dutch-style bikes are not for me. I like trekking bikes, which often have a good trade-off between speed, weight, stability, and lower maintenance than most road bikes (especially with belt drive and a gear hub). I also have a hybrid bike, which is lighter and faster, but I don't really like the amount of maintenance it requires (regularly degreasing the derailleur) and it feels less stable than a trekking bike.
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.
Though, if I could go back a few years in time, I would just buy something like an Urban Arrow. We had one last holiday and they are awesome for transporting kids + groceries.
It doesn't make much sense, in a way: If I have a trailer and 150-200kg of total weight (and I have as much every couple of weeks), why should 5kg more or less matter? But it matters: The bike feels better when it's not that heavily loaded, ie. most of the time.
What matters most of the time, matters.
So weight and speed probably do matter, even for regular people.