You must have a:
- Project documentation, coding styles, IDE set-up instructions and diagrams
- All the installation and environment set up instructions
- All the articles, video tutorials and everything else to help new team member understand an architecture and in case if any framework is used, how framework is different from codebase, probably, a lot of core components were changed
- Simple codebase and very simple installation process, I must be able to start coding after 5 minutes
- Good and simple management, communication and issues process set up
- Tasks to give to a new team member
- Architect, Tech lead
If you don't have something, then it must be done before bringing a new engineer into the team or whole team's productivity will go down.
Nevertheless, fresh minds are always needed, otherwise, there wouldn't be seniors to come from and senior would't be able to grow if they will need to code full-time everything themselves. Fresh minds also may give you many good ideas.
To sum up:
It depends and there are always tradeoffs in any case.