Boeing creating a new market?

Air Transport World today reports that Buckingham Research expects Boeing to forego reengining the 737NG, a move still anticipated by a "meaningful number of investors" in the case that a Boeing customer would defect to Airbus.
But Buckingham noted that Boeing might be so capacity constrained in terms of orders that it might not have the possibility to reengine.
Instead of reengining Boeing would move to an all new aircraft, probably a new light twin-aisle. As I laid out before, such an aircraft would not make sense starting in the current lower capacity end of the 737, so I would guess (if true) that this aircraft would start where the 737-800 is today. The upper end then could be where the  757-300 is, just below the 787-8. That would then fill a niche, because today there are no new aircraft build in this segment - well, one could consider the Tu-204, but this aircraft is almost dead and even not competitive with the 757.
Buckingham says that Boeing would continue to improve the 737NG and is internally convinced that the 2016 737NG is competitive with the A320neo. Boeing earlier stated that the 737NG line could be open until 2026.
A move described by Buckingham would be both smart and risky:

• Risky as it is not clear if customers really will think that a more than 40 year old design will be competitive. Until then the CSeries is in the market, a similar Embraer aircraft could be on the way to the market, making the 737-700 obsolete - the backlog of the -700 is shrinking anyway (as well as the A319). The cabin diameter will still be the 737's problem, as people tend to get larger - vertically, but even worse: horizontally.
• Smart as it opens a new market field for Boeing and by that getting out of the competition with Bombardier, Embraer, the Chinese, the Russians and, to some extent, Airbus. Although for now it remains unclear if the new aircraft will have transcontinental or transatlantic range, the aircraft could be (with the right level of technology) better than the A321neo even if it is designed for more range to enable all 757 missions flown today.

But is there a huge market for these aircraft? At the lower end for sure, as the 737-800 shows - but there would be the problem. If I have two aircraft in my shop, which one should I sell the customer. The same problem appeared when Boeing bought McDonnell Douglas and the MD-95 became the B717. The B717 had to die, as it was a stand-alone aircraft, not part of a family - Boeing sold the 737-600 to SAS although the B717 would have been much better in costs for the SAS missions.
And what about Southwest, by far the largest Boeing customer today? They just ordered the 737-800, but all of their other (500+) aircraft have less than 150 seats and they also like the B717 as an addition with even lower capacity. I cannot imagine Southwest going to an all 180+ seater fleet in the future just to keep being a Boeing customer. And I cannot imagine that incremental improvements to the 737-700 are enough to keep them from defecting to Bombardier (they might one day decide to build a CS500, although saying differently, Embraer or even Airbus.
There were "just" about 757's build - for sure Boeing counts on creating a new big market segment when they really build that small twin-aisle. The question is, if that market segment will be there when the aircraft appears...