Thats one phone ( and that stat is true in the US only). Marketshare is not determined by one phone only, when comparing platforms.
What that stat does actually tell us ( and Apple) is that a cheaper iPhone, with good enough functionality, can outsell any newer Android phone. If they hadn't released the 3GS on Verizon they would be leaving money on the table. If they had discontinued the 3GS they would be leaving money and marketshare on the table. Now imagine that they have a 3GS, an iPhone 4, an iPhone 4S, and an iPhone 5.
An iPhone 5 would be the high end pro model. In that model they can introduce a LTE next year. The iPhone 4S is the high end consumer, iphone 4 is the mid-level consumer, and 3GS is low end consumer.
I dont say they keep these names, but that kind of thing makes sense.
Eventually, loss of marketshare will reduce both of those.
Apple are well aware of this, they know the lessons of 1992-1995. There will be no repeat.