I have had my trusty 6s since a day after its debut in 2015 and surprisingly, I haven't had to replace my battery should I expect problems other than a battery that will eventually deplete to 0% battery capacity?
Well apparently the average iPhone owner keeps their iPhone for 4 years now:
Analyst: Average iPhone upgrade cycle now lasts four years, up from three in 2018
Analyst: Average iPhone upgrade cycle now lasts four years, up from three in 2018 - 9to5Mac
Toni Sacconaghi from Bernstein, who says that the average iPhone upgrade time has increased to four years, up from three years in 2018.
9to5mac.com
I think this would coincide with how long people may be happy with their iPhone up until begins giving them issues and they really feel the need to upgrade.
I know many who still have iPhone 6s, 7 and 8, they simply don’t see the need to upgrade as these devices do all they need.
I doubt you will face any major issues outside of the battery aging, most other parts of a smartphone can keep going for many years, the only the other parts that may go bad is the screen along with the parts that move, so that means your Ring/Silent switch and your home button to.