There is rebuilding and there is tanking.
Rebuilding is where you are creating an almost whole new team with players that you expect will peak in a future season.
Your team is still trying to win games as best they can, but you realize they are undermanned for the sake of acquiring future talent. You might trade a valuable veteran that would have helped you win more this season. However, one or two more victories would serve no real purpose for the current season.
Such an example would be the Cowboys, 1989.
Tanking is the deliberate attempt to lose games in order to increase the odds of acquiring better talent in the future through a more favorable draft position.
The problem with this is that willing management and coaching for that franchise does not value winning enough to the degree that they will most likely fail to build a better team. Their competitive spirit has been compromised and that will probably reflect in the type of players they draft.
Another problem with this is that no NFL player is going to tolerate such an attempt without exposing it. Their career, their persona, their self-worth is based on the uncompromising goal of showing the world they are the best at what they do. An NFL player cannot reach their level of success without the work, commitment, and sacrifice required. As such, they are not going to flush it all away on the possibility that they might still be one of the players in that team's future.
With 53 players on a team. it would be difficult for such a conspiracy to succeed without direct participation by the majority of starters at key positions.
To put it more bluntly, tanking does not exist in the NFL and rebuilding is visible for all to see. Rebuilding does not reduce the team's desire to win and sometimes they do. An example of this would be the Cowboys, 1975.