Because if/when the "new generation" of airliners debuts and it's got a significant cost advantage, remember, efficiency is market in single-digit percentage improvements to make phat loot for the operators, it will sell like gangbusters.
Remember, one of the only reasons you have a 737-800 is because Airbus jumped into the 150-seat market after Boeing decided the minimum airframe was going to be a 185-passenger 757 after the retirement of the 727. Then when the Frenchies came up with a 727 replacement that you could pull a plug out of and sop-up the rest of the smaller 737 (classic) lift, flow up to an tickle some of the 757 market and, believe it or not, be nearly type-rating compatible with a 330/340 with variants that compete directly with the 767 and the 777, I think they got caught with their pants down.
The idea that we're playing catch-up with the Europeans when it comes to commercial aircraft should be an American economic emergency. Boeing has to win this thing and it's going to take a lot more than re-engining 737's.
I am a huge Boeing fanboy, but I want our team to get their crap together.