Difficulty won't rise exponentialy forever. In a year it can be at 300M, which will make AM still profitable (energy-wise).
Difficulty does not have to rise exponentially forever. It just has to rise 35% month-over-month, for merely 12 months, and the buyers will never make their money back: https://bitcointalksearch.org/topic/m.1889860
For the record, 35% per month for 12 months would bring the network to ~2000 Thash/s total, which half of it is already being built or planned by various entities (100TH-Mine, BFL, ASICMINER, OEM Avalon Chip Customers, etc).