PostgreSQL isn't that special as a codebase. Every codebase has its quirks, every project has its own processes and there's a learning curve. When you switch to a new job as a software engineer, you pick it up. PostgreSQL is no different: you can hire an engineer to work on PostgreSQL.
I'm not sure how well that path works in growing new contributors, though. In a usual company setting, the goals are better defined, and the company is in control. Once you reach the goals, mission accomplished. With an open source project it's more nebulous. Others might have different criteria and different priorities. You are not in control. Choosing the right problems to work on is important.
Other storage or database projects would be a good source of new contributors. If you have worked on another DBMS, you're already familiar with the domain, and the usual techniques and tradeoffs. But to stick around, you need some internal desire to contribute, not just achieve some specific goals.
I'm not sure how well that path works in growing new contributors, though. In a usual company setting, the goals are better defined, and the company is in control. Once you reach the goals, mission accomplished. With an open source project it's more nebulous. Others might have different criteria and different priorities. You are not in control. Choosing the right problems to work on is important.
Other storage or database projects would be a good source of new contributors. If you have worked on another DBMS, you're already familiar with the domain, and the usual techniques and tradeoffs. But to stick around, you need some internal desire to contribute, not just achieve some specific goals.