In my opinion, the most optimal organizational structure to have is to train people who actively want to become managers (but don't promote them unless they can

In my opinion, the most optimal organizational structure to have is to train people who actively want to become managers (but don't promote them unless they can actually do the job), and give individual contributors an alternate career path - like that of a technical architect, senior contributor, etc. Being a manager is a very, very different mindset from being an individual contributor. And there ARE people who want the managerial career path too. The trick is in finding them and offering the appropriate training (because not everyone is cut out to be a manager). Lastly, the "manager" title is very broad. There are people managers, technical managers, project managers, etc. So training someone to be a manager needs to be tailored to the types of managers at your organization.