majority of airlines are grounding their fleet
Some perhaps, not a majority....
Wont this be sort of beginning of end for the high fuel consuming airplanes like 747?
And replace it with what? The 747 is still the backbone of many long haul fleets, and even if the airlines that have them want to replace them with say 777s, there is a not an infinite supply of them.
We're potentially in a position where things are getting harder. As a result, some carriers that are in trouble now may get into worse trouble. I refuse point blank to speculate on who they may be, since I would hate to add to anyone else's woes...
Asking which is the most fuel efficient aircraft is akin to asking "which is the best car". It has to be qualified with "what do you want to do with it". Eg, many would say a Ferrari is better than a Renault people mover, but not if you have to get six kids to school in it every morning....
The only real certainty is - the newer the aircraft, generally the more efficient it is. But remember that fuel costs are only part of total cost of ownership....
The key message in my view is that fares are going to have to rise. Will that have a serious effect on the numbers of people travelling? Jury's out. The growth in air traffic has been pretty much a straight line since 1945, through booms, busts, oil crises, terrorist attacks etc etc. More and more people are getting a taste for travel, more and more of the world can afford to, and more and more people are moving travel from "discretionary spend" to "must have item".