Has there actually been attrition? Exponential growth is enough to explain "many more juniors than seniors" at any time in the past, present or future.

Also for attrition to be the cause, you'd need a lot more seniors dropping off than juniors.

None of my friends who graduated with me are still software developers and I’m several years from retirement age.

There’s a bunch of filters. Many people quickly realize they don’t enjoy development, next is openings in management. One of the big ones is at ~40 you’re debt free, have a sizable nest egg, and start thinking of you really want to do this for the next 20 years?

A part of this is the job just keeps getting easier over time. Good developers like a challenge, but realize that the best code is boring. Tooling is just more robust when you’re doing exactly the same things as everyone else using it, and people can more easily debug and maintain straightforward code. So a project that might seem crazy difficult at 30 starts to just feel like a slog through well worn ground.

Having significant experience in something also becomes a trap as you get rewarded for staying in that bubble until eventually the industry moves on to something else.

The exponential growth has been something like 3-4%/year, or 2x in 20 years. Though it's hard to find useful statistics that take different job titles and changing nature of the industry properly into account.

If you had asked me in 2010, I would have said that the median software developer lasts 5-10 years in the industry. A lot of people left the field after the dot-com bubble burst. The same happened again in a smaller scale in the late 2000s, at least in countries where the financial crisis was a real-world event (and not just something you heard about in the news). But now there has been ~15 years of sustained growth, and the ratio of mid-career developers to juniors may even be higher than usual.