The "we've never seen it done any other way" problem is 100% a real thing.
One of my employers 60% of the company had spent all or most of their professional career there - and things were scrolerotic at times - because there was a huge amount of group think, and a strong NIH/"the way we do it is fine" problem - simply because of a lack of indexing to how other players did it.
You ideally want around 50/50, 50% lifers, and 50% new blood, its even helpful to encourage employees to leave go get knowledge elsewhere and then come back later if they desire.
One of my employers 60% of the company had spent all or most of their professional career there - and things were scrolerotic at times - because there was a huge amount of group think, and a strong NIH/"the way we do it is fine" problem - simply because of a lack of indexing to how other players did it.
You ideally want around 50/50, 50% lifers, and 50% new blood, its even helpful to encourage employees to leave go get knowledge elsewhere and then come back later if they desire.