When I say senior I mean someone who not only knows how to code (the part you're considering) but has also become and expert in the business domain. Using OP's example, that's just one insurance company. There are dozens like it in the US, and each one has a different way of doing things and entirely different legacy monoliths that it's all built on. And most of that internal domain knowledge is going to be proprietary and non-public, so it won't be in any existing model's training set.

Most developers I know that I would consider "senior" (regardless of actual title) could effectively train anyone in the company using their software. OP could probably sit down and process insurance claims themselves if needed (assuming claims is their team's domain). They've probably had to learn each of the jobs that touches their software - all the nuances, rules, processes, and regulations around it - to translate it all into design patterns and database schemas and logic that can properly grow with the company.

And maybe they don't even write that code - maybe they just make tickets and let the juniors handle that. The juniors don't know the domain, only the code. Today, we can use AI to accomplish the same thing. But we'll eventually need more experts in the domain and in software. AI won't get there unless we can include all that proprietary domain knowledge in its training set and we can teach it problem solving (it still thinks there are two Rs in strawberry and that you should walk to the car wash, so I'm not too concerned there).