This is a problem I have thought about over my career. The TLDR; You definitely need senior level engineers (Principal's & Above) as a career track in a healthy organization.

Here is why - (for the sake of discussion, "principals" refers to principals and above)

- In an org, lets say as a director, I find I rely on my principal engineers to objectively tell me what the right thing to do is. They have less political motivation.

- Principal engineers almost unilaterally have a series of noteworthy accomplishments that create a catalyst for innovation for all engineers around. The depth they create inspires people to really understand tech.

- Discussions with them require managers to have more depth themselves. Rarely can managers win arguments without brushing up on tech.

- Regarding some comments about "CRUD apps" as universal easy things all engineers can solve I find perplexing. Problems in distributed systems are all trying to make CRUD possible and organizations are still figuring out how to do that at scale.

- Regarding arguments about ageism for principals, yes, I agree that ageism exists to a certain degree. However, I find most principal engineers are older as you go up the chain. Therefore, what you are really saying is ageism exists for engineers < principal level. Where you can have new hires know the same depth as them and pay less for. Can you imagine trying to hire folks that built S3, Python, Kafka, etc. be replaceable by younger folks?