The phone is pretty durable despite criticisms of its plastic body. With water resistance it's an excellent long term phone. I've had my s5 for about 8 months and have not had any issues as far is the build quality. I've dropped it multiple times with a thin case or bumper case on it and dropped it in snow multiple times, wipped it off and have had no issues. Since it's not possible for anyone to have had the phone for longer then 1yr at this time it's hard to speak on how long it will last however, Speaking on Samsung products, I've owned a Galaxy nexus, galaxy s3, galaxy note 2, galaxy tab 3, galaxy mega 6.3 and Galaxy Note 10.1 2012 edition. The only product that I don't still have in my household right now is the Galaxy Note 2 because I sold it in excellent condition in summer 2014. The nexus was replaced by insurance 1 time because of water damage but the replacement has lasted for 2yrs. All my other devices work great and no issues whatsoever.
Chances are you'll likely have an issue with software updates or apps going haywire before you have a problem with the Galaxy S5 hardware externally or internally. Hope this helps.
By the way my brother has the HTC M8, excellent phone, although you can't change the battery.
Posted from my Monster Galaxy Note 10.1 via Android Central App