Earlier quoted context omitted.
I think people forget just how new the web industry is. The web itself has literally only existed since 1992. It's only been commercially viable to charge people for websites since around 1995. Web development in the modern 'building apps that are accessed in a browser' sense has only been done for about 20 years. If you're talking about 'single page apps' that drops to about 10 years. Someone with 2 years experience…
From a technical standpoint, I agree that good people can reach the bar of senior engineer very quickly - potentially right out of school. However, there are aspects of a senior engineer role that are not commonly learned in school or by hacking on hobby projects. How do you effectively lead a project? How do you balance requirements? How do you say no to stakeholders? How do you balance time vs quality? All engineer…
Moving someone up to a senior role is giving them the opportunity to take on more responsibility and develop their skills. Very few people are promoted to a senior role already knowing how to manage a team well or how to balance requirements. The expectation is that they've shown an aptitude and potential in a mid-level role, and that they'll learn those skills after they're promoted in to the new role.
Promoting someone to senior isn't a reward for already doing everything a senior does. If you treat people that way, and give mid-level people the responsibilities of senior roles without the pay or title, then they will leave very quickly to get jobs where they do the work of a senior with the pay and title of a senior.