I think it deeper than just a decline instructors.
I believe that historically you had a kind of pyramid scheme effect working. Lot of new young'uns coming through and a certain percentage of all new folk wanted to progress to instructor.
I'd be interested to see, but I'll bet a pack of peanut butter crackers and a coke, that the number of active divers worldwide is in pretty steady decline. And that of the active ones out there, the mean/median/average age (whichever you choose) is steadily moving up the scale. Same in the ham radio hobby.
Diving is an expensive hobby. Cost wise, it ranks right up there with flying and shooting. (How the hell did I end up with all three?) It takes time and effort to build skill. It seems that newer generations have little patience, compared to prior generations, for things that take time and dedication. Back in the 60s and 70s, couldn't wait for summer. Very little in the way of clothes, often no shoes, out the door right after breakfast, probably running or bike riding or involved in physical pay until the street lights came on. I don't see that anymore. Discussed with a neighbor (granted it's a limited sample) re: how his kids spend their days. They wanted to stay in the air conditioning, eating and either watching streaming movies or playing video game boxes or typing on their twitface accounts rather than interacting in social and physically active pursuits. Social media is not social interaction.
Is the bulk of the dive corps moving like a pig through a python towards the distal end of the snake's alimentary channel?
I am toying with the idea of getting a DM cert, not because I want to work in the show or lead dives in the last part of my 50's into my 60's, but to have an exclusive card and presumably recognition of skills/proficiency. Personal satisfaction accomplishment. Lot of folks I meet seem to assume I am one simply because of greying hair, how I carry myself and the knowledge I dog and seek out to understand the nitty gritty details of my interests. May as well check the box so when asked I can say "yep". And the training program can't hurt my set of acquired dive skills.
But no, I don't ever really see myself moving past that to instructing...