This thread has now become less about the proposed new tax and more about finding a solution to tanking prevention. that's OK, since it's closely connected. But there are problems here.
First of all, we cannot even agree what the definition of tanking is. What separates tanking from rebulding? Tanking teams from teams with no real opposition? Finally, as Sentinel remarked, it is a minefield of to navigate when trying to code a tanking "detector". Its not technically hard, it's incredibly hard to decide which prerequisites a team needs to fill in order to be declared a tanker and subsequently, be punished.
One of the fundamental problems is that most teams can only be competitive 2 games per week. But what when they want to win that cup game - then they have to "tank" one league match. Even then they might get pummeled in the cup and end up with two big losses in the week. Should they be punished for that? Of course not.
If we go the salary used vs total salary route, there are again exceptions and problems. There is currently no simple, clear cut way to detect tanking. In time, we might find a mechanism to do it, but I don't think it will be perfect nor simple. Until then, salary floor seems to be the best tool to discourage it, and I'd gladly increase it if there were no examples like the ones already mentioned here, where promotees have a hard time filling the existing salary floor.
So you see, it's easy to shout "just punish tanking!" when you're not the one that actually has to implement it.