SD is part of the reason for it's popularity IMO.
I think it was just the opposite. The fact that they chose to put Vanilla Android on a phone with SD despite how much they hate it is telling. They could just as easily have gone with the HTC One.
I agree with the first statement, but I suspect we differ on the degree. My estimate is that at most 5% of Samsung sales are due to any single feature, and on this one I'd guess it's less than 1% of buyers picked it for that as their primary reason. The rate is probably higher for people that considered it, but for that being a deciding factor, I think 1% is generous. My guess is that you think it's a majority of customers, but I think a majority of customers wouldn't know the difference or even what the SD card was without asking someone or looking it up.
I think the reason it wasn't (this still could happen) the One, is because the One, despite being an awesome phone, will not have anywhere near the visibility or sales of the S4. Again, I'd put the inclusion or exclusion of an SD card at approximately 1% of the decision making process; the rest being on features, flexibility, speed and politics.
Honestly, if you put your top 10 things to compare two phones side by side: screen, processor, ram, camera, battery life, lag, features, price, etc... is this actually in the top 10? If the S4 didn't have it, would you buy a dollar phone that did instead of the One or Nexus 4? As a total package the S4 is a great phone. That coupled with Samsung's success with the S3 and Note 2, + insane marketing budget, makes this a front and center device for mainstream consumers. Of course Google sees this as a win-win, and the SD card thing probably didn't enter into the politics much at all.