Having recently ridden in central asia (Kyrgyzstan etc), in my limited observation, the decline in bike "touring" is specific to USA.
I met and saw many Europeans (Brits, Germans, Belgians, Swiss and overwhelming majority French) starting their rides from their front doors in Europe, riding across Turkey, Georgia, Armenia, Iran, the "Stans", towards China and Mongolia with intended trip completion destinations in Kyrgyzstan or SE Asia. Ran into a few Australians riding the route reverse. Most of their rigs were sub $1000, 2x or 3x drivetrains, with the most expensive components being their pannier bags (which a few had acquired used). The well heeled amongst them had the $2K+ bikes and 1x drive trains.
Met two guys who were biking towards Mongolia and hoping to make Lake Baikal or Yakutsk before winter set in. One had his sight on Magadan, Russia (eastern Siberia) and I hope for his sake he makes it. It takes 2 weeks to get Russia visa. Only thing is bring US Dollars, not Euros. ATM and credit cards do not work in Russia at the moment
Age group of those I met early 20s to mid-30s. A few oldies like myself, but we were the minority. A few young women doing the route solo. Everybody was riding self supported and often solo, without satellite phones or SIM cards. Internet access they acquired when they checked into a hostel or home stay. Overwhelming reliance on offline maps from maps.me, since google maps is quite deficient (missing quite a few valid routes) in those geographic areas.
The roads outside the main capital cities are at parity or worse than official fire trails in US. Its a peanut butter mud show when it rains or snows. All riders noted drivers of cars and trucks didn't care to share the road with bicyclist, whether you were in Turkey, Iran or in the Stans. Cars and trucks belch thick pollution and you can often taste half burnt fuel. Drivers pass within a foot or two of your elbow. There is no highway patrol or 911 to call even if you had cell phone connectivity. Hospitals or health care facilities are a rarity outside capital cities. You will probably bleed out before timely aid can be rendered.
Most riders wild camped often as it appears to be permitted / tolerated in these countries. At higher elevations, you keep yourself warm by burning horse or cow dung (bring poop gloves). Sometime mountain melt caused these waist high water crossings across roads: wait until after sunset when melting activity is lower and water levels drop to knee levels making crossing easier. The water is ice cold no matter time of day or night. Nights are punctuated with vocals from wolves who are never too far away and the visiting wild horses that are annoyed with humans camping on their turf. Your self defense weapons are stones and your gut instincts.
When I talked to them about riding in Americas (North or South), they didn't have interest due to expenses (travel, visa, daily expenses) and impression of the Americas being unsafe. I estimated their daily budgets of an average of $10 - $15 a day, including food, in Aug / Sep 2024; its a rough living but not unhealthy. Many fly budget airline Pegasus that charges for drinking water !!!
GI sickness when in the Stans was not uncommon with few travellers being spared.
I could have done a better job organizing the thoughts from my experience, but then I would probably never get around to sharing. My point is old style (without satellite phones or internet connectivity, tires with tubes, and using simplistic bicycles) Bike touring is very much alive, unfortunately you will have to travel outside the US to experience the joy of observing others indulging in this wonderful activity.