When you think about it, a lot of players are gone from year to year. Particularly role players who are good enough to start, but not necessarily high quality players. A lot of those players will switch teams during the offseason without anyone really noticing because they're not really noticeable. A lot of o-linemen, d-linemen and safeties are like that . Typically, only the elite players who get long term contracts have little turnover. Most role players will have short term contracts, so every season you'll lose a few of them. So if you have a bunch of role players with contracts that are 3 years or less, a lot of them will be gone from the team over a 3 year span. Then you have to factor in key players who may have expiring contracts or get released or traded. Aside from a core of like 10 players like the franchise QB, main WR, main pass rusher, shutdown corner and whoever else is key to that particular team, a lot of players don't stick around for the long term. I think I heard the average span of an NFL CAREER is 3 years, so that says a lot. And let's not forget players who started, but lost their starting spot to someone else. Just thinking of my team, the Panthers, I can name the players who I remember were starters on the team three years ago and are still on the team. I may be missing a player or two, but this is off the top of my head and I'm only mentioning starters.
Cam Newton
DeAngelo Williams
Steve Smith
Greg Olsen
Charles Johnson
Greg Hardy
Thomas Davis
Charles Godfrey
Captain Munnerlyn
Jordan Gross
Ryan Kalil
That's eleven players who were starters three years ago and are still starters today out of 22 starters.