Yes, you are correct, for many airlines the remainder of the itinerary will be canceled if you miss a leg (and don't replace it). However, in some cases, it is possible to cancel a leg in advance (paying any change/cancellation penalty), possibly resulting in a lower fare...
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Couldn't you get the "Round trip, SFO-PVG: $700" ticket and then use it only one way PVG-SFO?
If you skip any segment of an itinerary, all remaining segments will be cancelled. In airline parlance, "flight coupons must be used in sequence."
Which is why you look for the flights DEPART->ARIVE->DON'T CARE and you just need make the second leg.
Don't do this often if you care about your frequent flier miles though.
Can you? I was told that airline tickets get canceled if you fail to complete a leg of the itinerary.
Yes, you are correct, for many airlines the remainder of the itinerary will be canceled if you miss a leg (and don't replace it). However, in some cases, it is possible to cancel a leg in advance (paying any change/cancellation penalty), possibly resulting in a lower fare...