This is the progression of life.
Im very philosophical in all that I do and I see these things as a progression.
There is a time when all things wane. As the people that once enjoyed these things wane, so do the things. Uncertainty in our futures, our advancing age, rising costs, less time available...etc, etc.
The PX10 thing hits a chord with me because not too long ago I bought a bike for its parts to put on a few of my bikes. The seller also had a 72' PX10. The SS I bought was his first bike and the PX, his second. He still had every bike he ever owned and they were all in great condition. He asked me to help him sell a couple of the others, and I tried for a bit, but there are no takers on great old road bike parts unless there is another reason. Greed and status.
If the PX was a fully equipped Campy bike, It would have parted out for several hundred. As it was, I looked up all the parts to see what they were going for and found just what you described. a Flood of unsold stronglight, etc. Even used Campy parts must now be perfect to sell them and the prices are going down.
The whole world is becoming what England experienced in the 1950's with bikes. Flooded with makes, models and parts so much that there is a glut, and not enough people to take advantage of them. That is why the English government forced a consolidation of the English bicycle industry in the 50's through early 60's. There were over 300 builders in the 40's after the war and non were selling enough to keep them or their employees in food and clothes.
I volunteer and work at the DM bike collective and there is no doubt that most people use a bike as status. They would much rather buy it new with monthly payments that get an even better one 2 years old for a 1/4 of the price.
Its real hard to enjoy all three aspects of bicycles for very long unless its your full-time life mission.
Buying, using and selling is very fluid and follows the ages of the people with interest. You can't ride a bike into your coffin. Even I at 58 am having issues. Im having to change positioning and saddles and clothes. This changes what I buy and sell also, as well as the bikes I do it on.
This is what the show American Pickers is based on

Eventually, its nice, and beautiful and cool as hell, but.... eh.
That all is a good part of what's happening but I think its a lot simpler than that. The buyers are there if the parts are dirt cheap, they're no shopping quality, they're shopping brand and cheap.
I often end up with bikes in whole lots at auctions, partly because it gets me what I'm after cheaper and it also gives me a way to make some of the cash back.
It seems every lot I've bought has a pair of department store three speeds, either Huffy, Murray, or Sears. (lots of Sears bikes). Most of the Sears bikes are the cheap American variety, not the better older Austrian models. I prefer to stick to Raleigh, Schwinn, Ross, Columbia, or any of the better Japanese brands but I often take what's there. There's rarely any competition other than a few scrappers in the crowd who bail out at $10 to $20 dollars.
I've can list a Huffy in as found shape and a Raleigh, put $100 on each one, and the Huffy sells first all the time. Most have never heard of Raleigh these days. They only know what's down at Walmart or Target. Old three speed seem to sell better than 10 speeds because most can't figure out friction shifting and two derailleurs. Most don't want gears at all. I also can't imagine why there's so many in their 30's who are buying their very first bike. Let alone how many adults I've met that never learned to ride a bike. On my street alone I'd venture to guess only one or two people even know how to ride a bike, and they span from their late 20's to their late 80's.
I think the drought in sales is that people simply don't have any cash to spend.
I keep hearing that companies can't find people to work but I don't see anyone hiring other than the fast food joints and a bunch of minimum wage laborer jobs. In this area we lost dozens of stores, most of the trucking companies, and most of our restaurants over the past year and a half.
During Covid lockdown, things got tight but people were spending those stimulus checks on bikes and toys. That kept up through last summer, then it died all at once here. eBay slowed to a crawl for sellers and now seems to have stopped all together. I see things listed that should be selling but there's no takers.
I've also found that a bunch of sellers have gotten screwy. I've had a half dozen sales cancelled after I paid with no explanation in the last few weeks and lots of items listed with best offer on them only to have the seller ignore any offer made, letting the overpriced item go unsold for months. I won a few items over the winter and had the sellers flat out cancel the sale saying it was no longer for sale???
About 1 in 10 CL sellers seem to respond, and almost no FB sellers lately. I don't know why they even list items there if they don't want to sell.
I listened to a guy the other day who started talking to me at Lowes. He went on about how he can't believe he can't find a good 10hp outboard this year, he said he gave his old one to his kid and had planned to buy a new one but they wanted $5k for one. He said he's been looking high and low but can't find anything. I told him I had just seen a half dozen listed on CL, all were four strokes and a few were over in PA where they're less likely to get full of sea salt.
He tells me he looked there but everyone was over 5 years old and they wanted BIG money for them. (Most were around $1,500, which is about the going rate for something newer than say 1990 or so. He tells me he's only looking to spend four or five hundred dollars tops.
No wonder he can't find one, he won't get a parts motor for that much, especially in the summer.
When you listen to people like that you realize what's really going on, half the people are broke, and out of work, the other half are just cheap.
Its worse with bicycles.
Every last one of the bikes I've sold in the past five years went to someone who was either here visiting someone, here for work, or just passing through. Almost never is it a local buyer. Lately even the long distance buyers have gone away, likely due to the cost of fuel.
I really don't know what I'm going to do come winter, my place burns oil and there is no gas here and heating oil was $9/gal the last i checked.