In addition to all of the other observations made (influx of cheap mutt rays, energy inflation, etc.), I think demographics are playing a role too.
As the Baby Boomers age out of the big fish hobby, I think the hobby will decline. I've seen quite a few boomer hobbyists retire from active big fish keeping. They get old enough that the maintenance is just too much work. Or they get sick or die. As until very recently the largest demographic, the boomers leaving the hobby is having an impact. Generation X is a far smaller demographic than the boomers and so can not pick up all the slack. Boomers tend to have more $$$ too, and are more likely to have the home space needed for big fish keeping.
As for the Millennials/GenY? I think they are in general less into fish and more into computers; further I think they are less likely to have the home space or discretionary income needed to keep big fish like rays.
Another factor contributing to the decline of stingray keeping is (I think) Covid. Covid produced a short but sweet burst in activity for ray keeping, and big fish keeping in general. As a side hustle I have imported Asian arowana and tank mates for 20 years, and I've seen demand for rays and big fish come and go repeatedly. But I have never seen a demand spike like occurred in the depths of Covid. With the government $$$ money and time on their hands stuck at home a lot of people either jumped into the big fish hobby or else indulged more. Now that we are out of Covid with a bit of an inflationary hang-over, I think a lot of people are jumping back out.
Will stingray keeping ever come roaring back to the glory days of 10-15 years ago? Given the changes in human demographics and the broader economy I doubt it. But I am sure there will be demand spikes in future. We seem to be in a bit of a lull now.