More butting head eh? I just see it as a difference in opinion. NW might be in a great financial position compared to other US majors, but they certainly aren't out of the water. We're coming off the holidays which leads to the long doldrum season of Jan, Feb and won't really pick up until March or so. That means no money. Until NW can resume to pre 9/11 operations and make the cash they were making last year I think we'll see more "penny pinching" aka good business practice. You are right though, that at some time those 9s will need replacing. True, it does take somtime to get an order off the ground with research etc... but NW traditionally secures all financing for new a/c at the onset of the order. What kind of credit can an airline get with the current financial situation now? Kinda like going into a bank with $30,000 in debt making $10,000 a year wanting a great mortgage rate and a loan for a $500,000 house.
As for the 318, I doubt that will happen. This based on the engine proximity to the jetway, and the fact that the 9s are flown on short haul flights, and the 318 is really designed for longer thin routes to maximize efficiency. The 318 in theory would be the perfect fit, but it doesn't seem it really is. Who knows, maybe NW gave boeing a great incentive to keep their factory open producing the 717... but hasn't made that info public yet.
We can speculate until the cows come home, until then, there is no immediate plan to replace the 9s, as they are one of the big reasons NW is in a good position now.