The way to truly lower fragmentation issues would be to only offer one device - the S4 - and allow Touchwiz or stock Android (and perhaps other skins) to be selected on a single device.
But as it happens, you have to choose one or the other. Someone could write software for the S4 Touchwiz edition but not the S4 Google edition or vice versa. That is fragmentation.
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So we should just have fewer stock because some people are stupid and develop to specific handsets?