I like aircraft with multiple wings stacked atop each other; in an alternate world (undergoing a similar technological history to our planet but with a different aesthetic) I would like them to remain dominant for both commercial and military purposes: multiplane equivalents for everything from 747's to F-16's.
Historically, multiplanes offered considerable advantages over monoplane designs; you have more lift with a narrower wingspan, and the rigging made the wings a lot stiffer as well. But from my (layman) research it seems that multiple wings are a cause of excessive drag at greater speeds, which became available towards World War 2 as engines became more powerful.
I wonder what I could do to combat that. My first instinct is increasing the air pressure*, which means achieving higher speeds is out of the question. That means that drag, which increases linearly with air pressure but quadratically with speed, is less of a concern, and multiplanes might remain viable; with monoplane aircraft restricted to experimental speed machines. Then it may take days for Average Joe to cross the Atlantic but with greater lift you can put more utilities like beds aboard a plane and have it remain a viable mode of public transport.
But increased air pressure might not be the best answer. A multiplane's increased lift may be a lesser advantage when everything has greater lift, including monoplanes. Hyper-manoeuvrable fighting craft would still like that edge, but no civilian crossing the Atlantic is going to want to fly in corkscrews.
My other instinct is the inverse, lowering the air pressure. Then lift becomes a greater concern and drag a lesser one, and stacking wings may be obligatory if you want to just stay aloft. But the more efficient solution to deal with lower air pressure is generally to just make your monoplane wings bigger, and as soon as there are materials as stiff as what's considered for the monoplane flying drone that works in Mars atmosphere, multiplanes would again fall out of favour. I'm fine with restricting branches of technology or materials, but "anything stiff" might be too broad a category of tech to rule out, as it would be too influential on the rest of the world.
So I am open to other changes. Take as your baseline the Earth in the late 20th century; what can I change to make a dominant multiplane design believable?
*The answers to this question deem increasing the air pressure generally survivable. Of course a planet with twice our air pressure for its entire lifetime would have lifeforms adapted to those conditions, so I am fine with exceeding the pressure tolerable by humans, as long as the resultant planet would still support a humanoid lifeform that looks the way we do.