Apple probably wouldn’t have crammed iOS 6 onto the 3GS if they were about to stop selling it. Marco Arment
I've been seeing a lot of fixation on the sale of the 3GS in the tech press lately and I think they're missing an important point: it's decent to ship updates for hardware that's under warranty.
The first update that didn't support the original iPhone was released 1 year, 11 months, and 10 days after the original iPhone was discontinued1. The 3G got its last update 1 year, 4 months, and 8 days after its demise2. If the 3GS were obsoleted by iOS6, someone buying a new one tomorrow would be getting the same kind of software support as if they'd waited until the last minute to buy a MyTouch 3G. People expect more from Apple than that.
The important thing about the chart Michael DeGusta posted last year isn't that every iPhone was on the latest software 3 years after release, it was that every iPhone ever sold has had access to the latest version of iOS for at least a year after purchase. Given the reported effort it took to get iOS6 running smoothly on the 3GS, it's not worth selling after the launch of the New iPhone—that would require getting iOS7 to run on it as well.