I don't think there will be so dramatic a drop off in interest after E3 that SW will just die at retail. A lot depends on the fatigue and commitment of the collector base, which has been large enough to sustain the line after disatrous flame outs before. SW would disappear between films at retail if not for this base; every other line does, and no other line has this staying power.
As for a third trilogy, I wouldn't mind it, but I can't imagine how Lucas would approach it. One thing I'm sure of is that SW will not disappear without a trace. Cartoons, TV shows, books, comics... it will survive.