New seniors are also newbies. I haven't seen a single senior who's been productive from day one. It always takes a few months to understand the systems organisation, get to know how people work, and so on. People expect 3 months to be enough, but it can take up until a year.

Regarding people staying and not making space, I'm not sure where the problem is. If someone gains skills, they get a raise. If there isn't money, that means you can't keep them around anyway because there isn't enough work to pay them. If you're still growing you'll be able to pay them and their increased skillset.

IMO there is no "space" issue. A team that starts out with 5 juniors and 5 seniors that grows into 10 seniors isn't an issue unless you can't pay for it. Am I missing something