I'm still using some evolved 2* girls, and it's fine. Most important part is: Can you 3-medal the event maps?
You can keep replacing your girls with more rare girls and get stuck with non-optimal teams for ages, waiting for gold and manyus, or get the girls you actually use up to about lvl 30 evolved (which is relatively cheap and easy to do with 2* girls), and then start replacing them with the girls you get from events.