Airbus has a significant advantage over Boeing with regard to the development of their A320 series of aircraft compared to boeings options with regard to their B737 series of aircraf.
Airbus can modify the A320 to take a new engine reasonably easily in that it has the space under the wing to fit a larger fan of any replacement engine. Whereas, Boeing cannot fit a larger fanned engine under the with due to lack of undercarriage height, the CFM56 intake had to have a very creative redesign to fit it under the B737 wing.
For Boeing to fit a new engine will require them to raise the aircraft undercarriage which is a major design and manufacture expense.
When all the modifications that will be required to the B737 it probably will cheaper in the long run to bit the bullet and build a new aircraft in toto which they won't be able to complete till round 2025 which leaves the A320 NEO in the box seat.
Nothing is impossible as long as there is both the will and, money
I agree with your analysis of the relative ease of fitting new larger fan engines to either the A320 or B737. However, your timeline for a B737 replacement is off by about 6 years. The current projected timelines are 2016 for the A320 neo and 2019 for a B737 replacement. There are pros and cons to either strategy. Airbus will be in the market first with an airplane that saves 10-15% on fuel burn. But fuel costs only make up 25% of operating costs on short haul aircraft vs about 50% on long haul aircraft. So, fuel costs aren't the only costs that airlines consider. If Boeing is able to address other cost savings such as maintenance as well as optimize their design for the new engines they will win in the end. It is unlikely that major B737 customers will consider switching to an Airbus product over a 3 yr wait especially if the wait results in even larger savings.
Fuel costs are by a significant margin the largest single cost of operating a aircraft and I do not think they will reduce in the near or foreseeable future. With increasing complexity of aircraft in all facets its going to take considerably longer to diagnose and fix a problem, even with computerised maintenance systems the unexpected will take longer to fix. When Boeing finally decide on the 737 replacement it with out a doubt it will be successful but I think, taking into account the 787 experience I suspect it'll be 2020+ before the 7X7 will be in general use.
One area in the operation of a aircraft that could assist in cost reduction, albeit a small percentage, is a reduction in the amount of paper e.g. flight manuals and on board maintenance manuals that aircraft and there crews must carry.