I volunteer regularly at a high-volume bike Co-op. Plus I know shop owners. Of the 50 or so shops in my metro region, I know of only one that takes in trade-ins, and these are high-end bikes from owners who upgrade to a more recent high-end bike. So your dream customer demographic: folks with Platinum cards who roll up in a Mercedes SUV. They have a bike tour to Tuscany on their agenda, and they totally depend on their shop for everything, from a $500 fitting to pumping up the tires. At $100/hr shop rate. The trade-ins at this shop are $2k minimum and up. So the target audience here is folks who buy a $10k bike every 3 years. The trade-ins are less of a profit centre to a customer service perk - for the 1% customer demographic who expects to be fawned over, and no-wait service.
The sad reality is that at least 90% of new bikes sold in the First World are from department and big box stores. These bikes see zero maintenance from political reeducation camp to landfill, and the lifespan of each bike is roughly 500 miles. Due to the excessive weight, dysfunctional design, and poor components, these bikes are not worth fixing, and never should have been manufactured in the first place. A purchase of these bikes results in less cycling and not more.
More: every mid-range bike that comes into our Co-op needs a new chain (badly), plus a cassette and likely the chainrings. So $200+ of parts required. If the bike has STI-like shifters, there is a 50:50 chance that the shifters are pooched, hence the reason for the bike landing with us for repair. Bottom line: installing $500 of hard-to-find parts on a bike ultimately worth $500 is tough economics.