Another article on experience:
https://www.footballstudyhall.com/2015/9/4/9254347/a-better-way-to-measure-returning-experience
An excerpt
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You need a seasoned secondary
Here are the correlations between different types of defensive data and change in Def. S&P+. Since I'm now using an adjusted point total for offensive and defensive ratings, one would expect a negative correlation -- % of returning ___ goes up, adjusted scoring averages go down.
Again, if I'd made wagers beforehand, I'd have bet on linebacker correlations being low. Everything I've read from others, and everything I've noticed myself, suggests that linebackers are a little bit less hard to replace than others. Or, the range between great linebackers and just fine linebackers is smaller than in other units.
This suggests that experience in the front seven isn't as big a deal as it is in the back of the defense. It is pretty remarkable that the correlations between returning DBs are almost as strong as those for the defense as a whole. I didn't see that coming.
And, for 2014, at least, the data suggested that the ability to get hands on passes was more valuable -- or at least, less replaceable -- then getting hands on the quarterback. I didn't see that coming either."
So it matches what we've seen. O-line is surprisingly easy to replace with new players.
Also, linebackers are easily replaceable where secondary is not.
QB and secondary experience with returning starters is most important.