Amazingly no one has brought up the declining sales of DSLR cameras. https://www.statista.com/statistics/799526/shipments-of-digi...

Flickr is great when, and arguably designed for, uploading photos from your computer. This after you dumped your DSLR's photos and edited them. The primary source of photos is now a phone, so services designed around that fare better.

Also, the professional photographer market doesn't really want to share all of their photos to everyone. SmugMug was better at allowing a photographer to control access to their photos and charge for access. Then the photographer puts a few up on Insta for publicity and advertising. Flickr was kind of caught in the middle, really designed for the pro-sumer DSLR owner, who are disappearing.