> then rather than promote you the org will simply hire people above you. And you will find that what you learned was not a solid foundation to move onwards.

Anecdata: junior engineer going through this process right now. Hard to move to a new company due to the specificity of the skills I developed and can't move up as they just bring in new people.

My impression is the exact opposite. As an intern, you want to learn from a company that actually has something to teach, not from one that cuts corners and is way too small, inexperienced and busy to spend time on guidance. But as a junior developer, you can grow with a fast growing company. Not everybody succeeds at that, but if there ever was a good opportunity for it, it's at a fast growing company.