Alternatively, you can take control and manually flash custom firmware. LineageOS is awesome.
It's a very safe bet that Samsung's current flagships are going to get Oreo. It might just take a long time, but that's something to expect if you've bought into the Samsung life. Their firmware is quite detached from vanilla Android, which takes time to add to new versions of Android. It also means there's less reason to be concerned with major Android updates, as Samsung is so different. May as well just assume any changes in future firmware will be minor, and enjoy the phone as is.
Custom firmware really isn't much of a mission. Unlocking the bootloader (if necessary) is a once-off step, flashing a custom recovery is done with the stock recovery, then just use the custom recovery to flash a root package, erase the phone, flash custom firmware and Google apps and clear the caches again.
After that the firmware can update itself. Pretty easy.
And if you really don't want to or you have a phone that isn't supported by LineageOS, don't buy a phone that is likely to be behind the curve if it's that important to you.