As I get older I understand the systems better but I get lazier. Give me 20 people and I will blow your mind, but on my own I get very little done. Part of why I manage people is I know what to do but can't actually do it myself (emotionally). Junior engineers are hungry and willing to burn themselves out working to learn what I know (like I did). I love love love junior engineers.
I haven't seen many junior engineers write code I was particularly impressed by, or that could stand the test of time. It's more tech debt, but it's also learning. Others here have said a mix of junior + senior is ideal, and I agree. But for some things, you need speed and low tech debt: those projects are when you should lean more heavily on your senior staff.
Junior engineers willing to burn themselves out doesn't bode well for the quality of work you're getting.
