Seniors can be mentored, too, and there are always ways to grow. A few examples: learning new languages/frameworks, applying AI, having more of a leader role, communicating more effectively, getting more involved in product, trunk first development, writing better tests, etc.

People shouldn't get the 'senior' title and just stop growing entirely (although it happens for some people)

This is the modern job market. Only hire people already hired and give them piles of money.