The vast majority of devs are under 30. Already most people are "senior" after 5 years experience. What are we going to do when this cohort wants to get promoted into architects and managers in 10-20 years time? There just aren't enough senior jobs.
This piece is to help people be marginally more effective and be one of the ones that gets it.
PS: your question assumes stable demand for software engineer, but I'm completely uncertain whether it will grow stay flat or disappear. It all will depend on how AI goes, as well and overall economic development and digitization of the economy.