I'm not sure that's true. The avg length of an NFL player's career is 3.3 years. Most will not last 5 years. Teams like the Patriots and Seahawks realize this and it's why they stay competitive year end and out. They trade away or let guys walk who many get surprised at in exchange for draft picks. They trade down to gain more 2nd, 3rd round picks.
The nfl is a constant reload league. You're not usually getting talent in any round outside of the first that last 5 years. If they do it's a bonus. With ballooning 2nd contracts and the rookie cap, your better off letting folks go and getting compensation picks.
This is why it makes no sense to draft a backup QB in the 3rd. Later in the draft, sure roll the dice on the next 'brady' but 2nd and 3rd round is what builds the core of NFL Teams.