The problem is, it's a bad strategy that made the game less fun for other teams in the league, and that's the point at which it becomes a problem.
In any event, tanking in the past has been a poor strategy -- when we studied it, we found that on average it was harmful, because the rate at which teams immediately repromote was simply too low (it typically needs to be around 80% under the old numbers to be break-even, all things included, and it's just not that high).