The Detroit Lions clearly have a plan and a mode. Apart from sheer talent, Brad Holmes and Dan Campbell want players who fit beyond their place on the depth chart. Campbell said it after the NFC title game loss, and during his season ending press conference Holmes echoed the sentiment that "everyone can't play for the Detroit Lions."
Certain areas of need for the Lions may call for a big splash in free agency this offseason, and if that were coming Holmes would not ever say so. Cap space won't prevent such a move, but the Lions also have multiple players in line for contract extensions this offseason and more generally in the near future.
The Lions have a core group of players that is set to stay in place for awhile. Priority is placed on retaining their own, as Holmes has freshly reinforced. But reshaping things around that core will always happen, and based on the success of the last couple season free agents will want to play in Detroit.
The Lions have 20 players who are currently slated to be unrestricted free agents. Some will be back barring something surprising, some are more questionable to whatever extent and it would be a huge surprise if some others will be back.
On that latter front, these six Lions free agents will not be back next season.