It sounds like the new teammates are too comfortable if their current learning rate is 2-4 years. That number could also be an overestimation, which reinforces the relative rank in the team.

This points to another "field of tension": when the value of the company is in the minds of the programmers (not in the code), how should an organization handle rank such that all team members become stakeholders in the most effective progress?

I suspect more like academic rank rather than "team lead" or "lead dev".