"Preparatory industry" is a bit much. The only thing people really need or use for interview prep is Leetcode, which costs about as much as Netflix if you buy an annual subscription, and provides hundreds of hours of what should be at least somewhat fun to anyone who actually enjoys programming

If you wanna get fancy you can throw in a $59/month Coursera Plus sub for those big-name-university math and CS courses. There are some good ones that I think would help people who focus exclusively on LC improve faster and/or get a deeper understanding

Ok, so we are now talking about ~$900 over the course of a year. Even throwing in some a la carte courses from edX, like the Programming Languages sequence from Washington that OSSU recommends, you can now give yourself a thorough and marketable CS education in a couple years of hard work for under $3k. That doesn't sound like a predatory industry....that sounds like excellent progress towards equitable opportunity

P.S. And you can even be a cheapskate and just use the Sedgewick booksites and do the UCSD and Stanford discrete math and algos courses on Coursera and get your interview prep done for a grand total of like $200

P.P.S. The reason you don't need algos in your day to day is because you are not good enough at algos to get a funner, better paying, more impactful job where you do