Very happy with my S7 & S7 Edge, performance does me good with both devices being very stable and reliable.
As to "why not upgrade to S8/S8 Plus?". For me tis the screens 18.5:9 aspect ratio, the long-n-skinny approach (like my phones with a bit more meat on their sides), and the location of the fingerprint sensor... Oh yeah, we want to move away from the curved display, looks pretty just not my thing we are finding.
Plan on waiting anyhow until 2018 to see what unfolds with the phone manufactures.