Also a lot of people who jumped on with the S3 saw basically the exact design ethos rinsed and repeated with the next two s series phones. They're bound to jump ship in search of something fresh and less "meh".
You can't speak for all. I went from the S3 to the S5. My GFs S3 upgrade is due soon and she may get my S5 as I'm I'm looking at the Galaxy Tab S 8.4, which is basically a blown up S5. Meh is what Apple did phone after phone...
My view is that any phone you get just becomes another phone very quickly after the novelty wears off. I've had no issues with the Galaxy range so why not stick with it, even if the improvements are incremental. Especially when nothing else is jumping out that is a must have.