It's definitely a mix. You can't hire just juniors because you need seniors to mentor/train them. You also probably don't want just seniors because as you said,

It's definitely a mix. You can't hire just juniors because you need seniors to mentor/train them. You also probably don't want just seniors because as you said, they need the bodies. Add to that the fact that you must expect people to leave, so you have to plan for the future. If you can manage a balance of both, depending on the individuals in the team it can end up being beneficial as juniors level-up and seniors tackle the higher-level stuff.