This is a serious thread.
Miners are going to collude to offer free transactions? That's a serious thread?
That isn't the attack described in this thread.
The OP is about a cartel (or cartels) withholding their customers' transactions from the other miners.
From the OP: "They could even offer 0% transaction fees (even refund mandatory tx fees) to entice more of the masses to process through their servers."
That is an optional mode of doing the attack. It is not required for the attack. It may help them accelerate the attack by drawing more customers to the cartel.
1) Why bother withholding the transactions if they're free?
The other miners wouldn't process them any way (when coin rewards are near 0). The effect is still to withhold fees and revenue from the other miners.
2) What are "mandatory tx fees"? Are you confusing the behavior of the clients with the protocol?
I said the attack would still be possible for a coin that decided to make transactions fees mandatory. In bitcoin transaction fees are voluntary and variable, but I was just preempting the incorrect argument that mandatory and/or fixed-rate transaction fees would be a fix.