So essentially your logic is nobody should ever get an extension, because they could always get injured after the fact.
Ultimately Diggs situation is nothing like the others you mentioned. Tank (who is an exceptional player and busts his ass every single play) Jaylon, Gallup and Steele were all coming off significant injuries at some point (some right before the contracts). Zeke's play warranted a contract extension, but not when they gave it to him (they did it with 2 yrs left on his deal). Diggs had been completely healthy and an absolute all-pro, he was the league's elite playmaker at CB. He was due for a contract extension and earned it. He got injured after the fact.
You can't predict injuries. In Diggs case he tore his ACL in practice.
I don't think a team can build a consistent winner without keeping at least some (if not most) of it's best, most proven players. You arnt going to get bargains in free agency by signing other teams' guys, and that means you are under immense pressure to basically nail every draft pick to churn the guys you lose. Maybe some team will try it some day and we'll get to see, but I don't think it's possible.