This lie is the one I am currently addressing.  In this scenario you have somehow convinced 51% of the miners to jump to the new alt coin in addition to creating the necessary alt nodes, wallets, exchanges etc.

To do this you must have convinced these miners they are going to make more income on the alt chain than on the Bitcoin chain.

Now you are going to turn around and ask all these same miners to take a loss in revenue and go back to hashing on the Bitcoin network with the hope of performing some sort of attack?  Not only does this leave no hashing power to run the new alt coin, if your attack works and the value of BTC drops because of it you are asking these miners to take a total bath on all the Bitcoins they mine during the attack.

Getting 51% to join some sort of centrally UN controlled inflato coin is a total pipe dream.  Getting 90% to join is not even worth talking about.

BTW: the idea of a centrally contolled coinbase subsity rule requires a central point of control which in turn is a weak point in the system and an obvious point of attack for those that don't like the idea.  In other words a distributed Bitcoin network with half the hashing power is still more secure than the centrally contolled UnCoin due it its central control.