You know that already happened? Like 15 years ago was what it looked like before every started practicing, today most people are practicing. If they fail to take your job today then it is because they failed to pass the interviews even with extensive practice.
If the tides shift and big tech starts tightening their belt, leetcode practitioners will start competing 100 engineers for 10 positions at big tech, and that's when salaries get pushed down.
Most books and blogs on professional growth say to have some sort of talent or skill that makes an organization need you. That way, you aren't fungible and have leverage. Right now my "skill" that I demonstrate is leetcode and the only leverage I have is that big tech needs more engineers and have to bid against each other for my fungible work. The moment tides shift, myself and every other leetcode practitioner are basically fucked.
When will this happen? Ask a fortune teller.
Anyone who has taught introductory CS courses at an average public college/university (so, not a huge R1 state flagship) can tell you that getting into programming is not easy. Plenty of students who I would describe as "smart enough" have an incredibly hard time learning how to program.
The bar is a lot higher than most people here seem to believe.
it's already like this just for positions at random software companies paying $80-120k. the percentage of applicants to google that end up getting offers is estimated to be in the low single digits (if not below 1%). and frankly, I suspect big tech companies already hire more engineers than they "need", just to deny them to rising competition.
I don't think you're wrong about the moment that the tides shift, but it seems we are still quite far from that.