Never, ever, ever intentionally throw a game in fantasy football. It's TERRIBLE for the competitive balance of the league. Ultimately, having a well-functioning, acrimony-free league is far more beneficial than trying to earn yourself a better round 1 playoff matchup.
This debate comes up every year. And the best that anyone has ever come up with against tanking is either:
A. Sportsmanship
B. Karma AKA Fantasy Gods
It is this simple, the two bubble teams do not control their own destiny. Their season, roster, schedule put them in that situation in the first place. No matter whether the guy tanks or not does not change that one of the two teams is eliminated. He earned the luxury of determining his own destiny of sorts and unless there is a league rule against it he should do what is in his best interest not the interest of two league mates.
He simply doesn't need to win this week and he has earned the right to better his chances for next week.
For all we know, his losing could also improve his waiver position. There a lot of good strategic reasons to tank. Thay are more logical than the emotional tirade of it's "bush league", "think of the league first", or "the fantasy Gods will smite you".
If next year, he is in the opposite position he'll understand the situation, and while upset, won't hold a grudge because it is a part of the game.
You forgot one.
C. Social Reciprocity.
As societies, we enact all sorts of social codes that discourage certain behaviors that would be advantageous on a micro scale, but disadvantageous on a macro scale. For instance, if I'm in the grocery store, it's in my own best interest to cut in line. It gets me out the door faster. It's not like there are any laws against it. The worst consequences I'd be faced with are being made to go to the back of the line, which is what would happen to me anyway if I didn't cut. So, best possible outcome for me is that I cut in line.
At the same time, if everyone in the grocery store reached that conclusion, and everyone tried to cut, chaos would reign. All of a sudden, my advantage from cutting would be completely negated. Everyone fighting their way to the front would cause all sorts of delays, meaning if everyone tried to cut, EVERYONE would get out of the store later. This is a situation where, when everyone acts in their best interest, everyone is worse off. Because of that, we've created a social code that valorizes waiting our turn in line. We all decide to act against our individual interests with the understanding that we all benefit in the long run.
Tanking is the same thing. Yes, in some individual situation, intentionally losing a game might result in a better outcome for you. At the same time, for every such situation, there's another situation where if another team tanks, it will result in a worse outcome for you. Fantasy football is necessarily a zero-sum game- one team's win is ALWAYS one team's loss. If one team's situation improves, everyone else's situation must by definition worsen by an exactly equal amount. So, if we imagine a no-holds-barred fantasy world where everyone tanks when it is most advantageous, in the long run, nobody will actually benefit from it. Today's gainer will be tomorrow's loser. What that crazy, tanktastic alternate reality WOULD accomplish, however, is hindering the goal of teams reaching the playoffs and getting seeded based purely on merit. So not only does no one benefit from tanking, but everyone who believes that fantasy results should reward merit (which, hopefully, is almost everyone) winds up losing.
Situations like this are why reciprocity exists. I agree not to tank today in the hopes that you will agree not to tank tomorrow. Competitive balance remains. Dystopia is avoided. Social mores win again.
Never tank. Ever.