Junior engineers probably aren't as appealing to FANGs. You have to train them, and teams built of senior engineers may not want to put the time into doing the training.
Junior engineers are very appealing to most big tech companies because there are lots of them and they need to hire lots of engineers. Also, the smartest engineers with zero years of experience are all looking for a job. The smartest engineers with n > 0 years are mostly happy where they are.
The main exception AFAICT is Netflix, who really prefers not to hire new grads.
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.
Don't forget internships.. it's essentially a hiring pipeline that is only available on students..
It's a lot easier to get hired when you are given a multi-month internship to prove yourself. Instead of having to prove yourself in 5x45min interviews.
(On the other hand students don't have experience to draw on)
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.
