inflation adjusted diving has gotten cheaper that it was when it was growing as a sport in the 70-80's, it's a really bad argument to blame it on cost.
I think part of the problem is cost, but not in the "the total cost is way too high" sort of way.
I have probably 15k worth of bicycles in my garage right now, but when I got back into cycling/racing, and I bought my first bike in years, I knew that at a minimum, I was buying something that had some utility outside of the sport. I could ride to the store or whatnot. I didn't have to use it for racing.
It's the same with running shoes - people try new shoes all the time, but if they don't work out for running, they still have utility. I can use them to walk in, do yard work, etc.
Scuba gear isn't any more expensive than a good bicycle (I'm not talking about Walmart bikes here), but outside of diving, it doesn't generally have any utility to the consumer.
There are probably some other things at play too:
1. Our society is getting older, so there are fewer new customers to be found.
2. Scuba isn't instantly gratifying - you have to do the work first. (PADI does have a model for addressing this, but it's not really applicable unless you're a resort, or you have your own pool).
3. Kind of like the previous, but you can't just go jump in the water. You can jump on a bike or go for a run because you know how to do those things.
3. Scuba isn't really a sport. You can't get fit from diving. It won't make you lose weight or get stronger. There are no real competitions between people that are performed on scuba that are generally available to the public, or that appeal to a wide range of people.
I think learning to dive is more like learning to fly. There are technical and medical aspects that are similar (though the bar to entry is lower for flight training than it is for scuba). There's an "adventure/cool guy thing to do" aspect.
Flight training's doing fine though. They keep certifying more students every year. The scuba industry's churn and burn model is self-defeating. All the quick classes with overweighted students work great if you're a dive resort and you aren't going to ever see those students again. Resorts don't have to worry about retention, the customers just keep rolling in.
Meanwhile, the divers they train don't have the skills to enjoy scuba diving, so they move on to something else.
When I worked for someone else, that shop had the same churn and burn model. Get them in, sell them a mask, snorkel, fins and boots at a huge markup... about 10% stuck around for other classes or bought gear.
Working for myself, almost all my customers buy gear and more training. We don't have to sell them on anything because they're getting quality training and we don't treat them like numbers. They want to continue diving, so naturally, they take classes and buy gear.
There's a bunch of poorly trained, certified divers out there who would be making shops money, if they'd been taught correctly in the first place. My kid's orthodontist and his wife, for example. Both were churned and burned by a local shop. She had a major panic in open water during a mask skill and her instructor held her under the water. She made it through the course, but now neither one dives because she has said she'll never go diving again. He wants to go, but his natural buddy learned to hate diving.
If we want to fix the industry, that's where we need to start.