I pledged to not write another part of the "...successor" story, so I named it differently. Scott Hamilton once again broke the news: Boeing is now leaning towards reengining. If that would really happen, this would be what John Leahy always expected (and so did I).
If we recap all what has been said before it becomes clear, why a reengining would be the best possible action for Boeing. Of course, a new airplane could be better by about 10% in operating costs, but at what cost and risk for Boeing? Just look at another story published yesterday by Scott Hamilton about the latest round in B787 delivery delays! It will take a few years for Boeing to get in a position where they earn some money with the B787. And with the refined A350-1000 Boeing might now feel the pressure to do something with the B777 earlier than planned before. The Y3(B777 successor) was currently planned around 2025, but with the A350-1000 arriving in 2017/2018, that could move to somewhere around 2020. That moves the Y1 (B737 successor) to the second half of the 2020's - and meanwhile, say in 2017 a reengined B737 could appear. Capital requirements for that, if true what Hamilton writes, should be in the same $1-1.5bn range as the A320neo program - probably a little less than Airbus's costs, as we can assume that we will only see one engine on the B737RE - a LEAP-2B, how I would call it for now: with a smaller, scaled down core, a fan with the maximum possible diameter to fit under the wing without nose gear extension.
The technical risk is minimal, compared to the risk of a new airplane a la B787. Additionally, Boeing will see if the LEAP engine will work as anticipated when entering service on the A320neo.