Tanking a season is not the actions of a team of players or their coaches. The ability to tank a season comes from management, particularly the GM or person in charge of player personnel.
First, from a common sense point of view, if I am a player, do I want to play with less intensity just so the team can draft a better player? What position will this higher drafted individual play....mine?
From a coaches point of view, these seasons are my resume. Common sense tells me I'm not going to be coaching here forever, no matter how the current team performs. What will matter to any future employer will be the overall performance of my teams compared to their perceived talent level.. Why would I want them to perform at a lower level? I'm already challenged by their lower talent level.
The tanking would have to come from those that decide how to manage the talent level. They would be the only ones with the authority to change the team's overall talent level by manipulating the roster.
It's important to note that their intent isn't to lose more games as much as it is to increase the probability of winning more games in the future. Tanking is a form of trading current success for future success. It's not really as insidious as it sounds.
For example, after the end of the 1988 season, the Cowboys had a really low level of real talent. Most of the talent they did have had declined through age and poor drafting. However, they had one great player, All Pro RB Herschel Walker. Walker could improve the team's overall 1989 record by a few games for sure but it would never have been enough to make the team competitive. The Cowboys would have to lose for several years as they acquired talent through their own draft choices.
So the Cowboy traded Herschel Walker as a method of accelerating the increase of talent level. Of course, the repercussion was a huge decrease in the talent level of the current team. However, it did not reduce the the effort of the current team. In fact, I would say it probably increased it, although the talent level of the team dictated that the effort would not be enough to win games. Every player knew that Jimmy didn't care who you were, if you weren't working hard enough, he would get rid of you. The team was never going to tank by decreasing the individual efforts of the players.
However, it was clear that trading Walker was going to reduce the team's success in 1989. Wasn't this tanking? In Jimmy's capacity as GM, because he did have total authority at the time, he reduced the team's ability to win a few more games. The answer may lie in the way we define "success". Is going 3-13 more "successful" than 1-15? Technically, yes. If, for example, you are in debt by a thousand dollars, then you are better off financially than if you were in debt by ten thousand but you are still in debt.
Here is a question....If I trade a draft pick for a much better one the following year, am I "tanking" the current season?
To summarize, tanking is a myth in the NFL. There are too many players and coaches for a team to conspire to deliberately lose games through less individual efforts. What really happens is the team's executives will make a decision that they believe will result in more success over a specific time period...like the contract length of a GM. The side effect of their decisions will detriment a current season, although GM's would prefer that they don't. The objective is not to lose games and get better draft picks, it is to exchange or liquidate elements of the team that will not be contributing in that future in order to replace them with elements that will be contributing.