It’s been more than three months since I last wrote about our need for a financial transaction tax on the city’s exchanges. But in that short time Mayor Emanuel’s given us roughly 700 million new reasons for supporting it.

My inspiration in these matters is Joan Kufrin, a retired freelance writer from Chicago, who had a revelation earlier this year about the potential benefits of the so-called LaSalle Street tax as she watched Mayor Emanuel inch closer to her pocketbook. (Metaphorically speaking.)

This is a tough argument to sell. For one thing, traders are supposed to be the masters of the universe, the last true believers in the free-market. By begging off this tax they’d in effect be forcing the rest of us to prop up their industry by allowing it to continue operating tax free.

In other words, we won’t raise any money with it anyway, so why waste our time trying to implement it?

The sad fact is that this excuse hasn’t been relevant for years. The big exchanges—most notably the CME Group—are cutting their payroll as they move toward electronic trading.

Alas, I’m enough of a realist to understand that the transaction tax won’t be happening anytime soon, no matter how much sense it makes.