The other thing to consider is how teams get depth. They generally dont SIGN valuable depth pieces because guys that are walking the streets usually are doing so with the hopes of starting somewhere. Depth comes from young competition and aging veterans. Players capable of contributing in a pinch but you don't want to depend on them for an extended period of time. No team has 53 starters on their roster, because if they were at that level they wouldn't be backups. It's a constant rotation of depth. As a 'better' player comes in, that current starter either leaves, or falls down the depth chart. This front office has had 2 drafts / off seasons that have demanded more than half the roster be signed / re-signed. They've been effective enough drafting, that most of their picks are starting, or forced to play. Arguably our weakest position group, we've drafted more players into than any other position group in CB with Amos and Sainristil.
Depth is a moving target, it's very rarely consistent from year to year because players rarely (and never should be) are content with being the 3rd DT / DE, or 5th DB / LB. They're constantly trying to improve, and 'hope' they get a shot to prove themselves, and when they do, they aspire to start whether it be here or somewhere else in free agency. Guys don't sign 4 year deals to be a backup at any position other then maybe QB. Good teams generally lose a lot of players in the off season because of performance, and bad teams generally let them walk willingly.
I think it's more of an endorsement that even if a team 'under performs' in a given season, continuity is key. When teams get good, they are faced with the polarizing question of "do we pay the stars and get top heavy, or do we spread the depth and try to raise the floor?" Teams like KC, Buffalo, and Baltimore let premium talent walk at other positions, while teams like Philly and Detroit invest heavily in the skill positions and raise the floor. All of them continue to build a certain way, and do so with a plan. They keep their staff together as long as they can, but the direction of the players / roster remains the same, so it's easier to get guys integrated at depth positions as they come in young in hungry, or join in during the descent of their careers.