There will always be fewer people at the top of the pyramid than there are at the base. As such, there will always be too few opportunities for everyone to grow

There will always be fewer people at the top of the pyramid than there are at the base. As such, there will always be too few opportunities for everyone to grow and/or be promoted, especially not within a specific timeframe of a few years. A company is not an ever-flowing pipeline up that pyramid, the job still needs doing. So it's normal that indeed people will leave to explore other opportunities in other places. I'm not certain this can be resolved in smaller or medium-sized companies. The amount of opportunities tends to grow with the size of the company.