I still have SE W800 (close to 7 years old),
Nokia N85(3 year old). If SGS was with me, its 2+ years, never changed battery of any phone
Phones do stay for long if used carefully!
i still have my first phone nokia 3310 in working condition , but i dont use or cant use it
phones will stay alive longer but the urge to use them wont be , so dont look too far and spend too much
just plan something for 2 years ahead and buy something from current trend and let it become old after two years so u will switch to the latest
who knows wat will happen in 2 years , flexi display , laser keyboard etc etc anything can happen
S3 isnt a magnificent device considering the current trend now , like it was when S2 got released or when Note 1 got released
get something new and something reasonable !!