Unless your business is growing like crazy, the best way to measure the influx of skills and ideas is through your attrition rate. Most companies think it’s good to keep it low (5 to 15%) because they don’t have to retrain new employees and they retain the knowledge base.
But a 10% attrition rate means that it takes 10 years to replace the department. You are not going to be getting a ton of new ideas through the door and elevating to a level where they can impact the business. The word you are looking for regarding this condition is: stagnation.
Stagnation is when things slow down and the internal team is mainly focused in on themselves and what they’ve always done instead of looking out for best practices. An internal focus can be a short-term benefit (which creates long-term confusion) because it allows for ideas to fuse and really come together. But after a certain point this benefit goes away because you can over focus on ideas.
It’s all about a balance – don’t be too high. But also don’t be too low.