The damage from smartphones was done a while back. The truth now is that Nikon (and Canon to some extent) failed to be at the forefront of: (a) action/drone cameras, (b) 4K video recording, and (c) mirrorless interchangeable-lens cameras. Sony and Fujifilm have seen a real renaissance in their mirrorless camera lines (I switched from Canon to Fuji and haven't looked back). Sony has small mirrorless cameras that shoot 4K on par or better than Nikon. GoPro created a billion dollar public company yet Nikon only just recently announced their own action cameras.
I worked in the photo industry for several years and Nikon was once the gold standard. IMO they have no one to blame for their current state than themselves.