The problem is finding the right people to replace them though. I'm a decent programmer but our architect is an amazing one, understands security, performance, the underlying infrastructure way better than I do and so on. It'd be incredibly hard to replace him since there's not many people as skilled as him so that's a challenge from a technical point of view. It's really tough, even with a decent knowledge transfer period.
So, having 70% of the original team leaving will make the overall technical skill go lower, at the very least in the short and medium term. Along with them game design knowledge also leaves along with office culture, so the type of games the studio were capable of producing are at risk of not being possible anymore.