I was a member for a number of years; I thought supporting the organization was worthwhile, and I enjoyed the magazine. Then the mission started to diverge from supporting and encouraging long-distance bicycle touring, and a few years later the magazine went to heck in a handbasket, so I let my membership lapse.
A few theories:
One, extreme adventure has drunk too much Mountain Dew. The AT and TransAm are just as hard as they ever were, but they're too tame to brag about. It seems like cross-country cycle touring and blogging about it (public visibility) peaked roughly 2005-2015. Early in that timeframe, someone speculated that 2,000 people a year started to hike the Appalachian Trail, and 200 of them finished it; while 200 people a year started to ride the TransAm, and most of them finished it. By the time I rode the TransAm, I'd guess the cycling census was 1,000-1,500 per year, from cgoab journals with liberal extrapolation. This year there was a small handful of journals there. Of course, the propietor went off the rails, IMHO, and drove a number of people away, so there could be many more on one of the other social media sites, or just not blogging for public consumption. Meanwhile, the AT traffic, according to my little sister who finished it a few years back, has exploded. Now there aren't trail angels, there's a trail service financial system to shuttle people off every road crossing, put hikers up overnight, etc. And when the AT is conquered, it's time to move on to the Pacific Crest Trail, and try to finish it around wildfires and smoke. Or go to Africa, Asia, or South America for even more extreme adventure.
Second, as a few people have noted, after you've done one tour, online maps, web searches, and smart phones let you plan and re-plan a bike tour without the need to take a pre-mapped route.
Third, I've had a pet theory for a while that specialty magazines run out of new things to publish after a few years. Examples? Bicycling magazine, for one. Subscribe for two years, and at first you're drawn in by New! Product! Reviews! and century training plans. After a year or so, you start noticing that you've read these articles before. Bicycle Quarterly, because it's published quarterly, took two or three years to hit that spot, IMHO. They then pivoted from quasi-scientific tire reviews and racing brevets to (could you guess?) tours in Africa, races across Europe, and touring in Japan. Fine Woodworking was another hobbyist magazine I followed for a while; after great articles on how to make great woodworking joints, design articles using those joints, and finishing the pieces, they slithered over to making your own plywood so you can build extremely bent wood pieces. Adventure Cyclist had some great columnists for a while to supplement the "I rode across the country" articles. John Schubert retired, Sheldon Brown died, Aaron Teasdale moved on, and they honestly haven't been able to replace them. Now the magazine has to fill its pages with "We rode across the country with X" articles, where X is a disability, different gender, racial diversity, etc. You can only inspire with the same story so many times.
Fourth, the organization has lost its roots in volunteer or founders' leadership to professional non-profit leadership (if that's not a contradiction in terms). There seems to be a playbook -- or maybe it's a management course -- in how to run a non-profit. First, fund-raise from large money sources. Second, look around for outside-the-box revenue streams; these can be open lots of stores (REI), or look for new audiences or tours (AC). You plan on how to survive declining revenues as your legacy programs wilt, and hope those new revenues will pick up before the organization goes out of business.