I think Wizz has summed it up quite well!
They will still be around for many years to come, but the major players will slowly phase them out as they replace them with more modern equipment, the operators slightly lower down the food chain will pick them up doing charter work because they will be incredibly cheap to get hold of, and of course more and more of them will end up in cargo operations to start their second life!
As regards the 737-NG, well I have flown both 757 and 737 (including NG) and the thing that makes the difference is the fuel burn and hence part of the operating costs for my carrier.
I fly holiday charter ex-MAN. The RAMP FUEL for a 737-800 to fly to the Canaries, with 189 seats on-board, is about the same as the BURN for a 757 to do the same flight. Our 757s are configured to carry 235 pax, so that is a lot of extra fuel for those 46 passengers...
Now that doesn't mean the 757 is no good! Far from it. It is somewhat over-powered, which means you are rarely limited, even with smaller airfields and long distances and as for the brakes, well, all those extra wheels leave the 737 standing (well, actually they leave the 757 standing, whilst the 737 sails passed!) And the cockpit is big enough to hold a room party in, which is more than can be said for the 737.......
So from a piloting point of view, I think the 757 is the nicer place to work (and it's CAT IIIB capable), but from an economical point of view, the 737-NGs have the upper hand.
PP