but our solution as game designers to this problem is that such teams are very likely to get relegated.
These teams affect the outcome of the regular season in their current league regardless of what happens to them in the short term (relegation) or in the longer term (they bounce back stronger or stay down and end up losing the financial advantage they gained). It's not fun for their current league mates, and it doesn't seem fair to teams they will face the next season.
I haven't seen any proof that tanking is a winning strategy. A team which is bound to relegate (100% chance) is probably better off tanking, but any team that has a chance of staying up needs to factor that possibility in somehow. However as long as it is possible to make a large weekly profit, some users will try it. In doing so, they affect the competitiveness of their league.
My league (my conference in fact) currently has two tanking teams, which is pretty boring for the rest of us. I appreciate the fact that these guys had reasonably competitive lineups for the first 7 games or so, which means teams in both conferences had practically the same number of easy games. It would have been pretty sad for the league if they had tanked from game 1.