Can't replace my battery? Only option is to buy a new iphone?

I am really frustrated with my current situation. I own a iphone 6s and want to use it for another year before i upgrade but the battery dies at 10%. Coincidently i found about the battery replacement incentive program so i quickly jumped to it. I Waited over a month for the call from the local apple but didnt get an one until i called and schedule it myself. After 2.5 hours of wait at the apple store they told me that my phone can't be open and they can't replace the battery. They said there is a tube missing when scanning the iphone and without the tube it is unsafe for the technician to replace it. The store manager apologized they offered me to buy a new iphone 6s at 30% discount.

This is the most rediculous thing that i have experienced with apple. They keep aruging that someone must have opened the phone and took out some part and therefore they won't open the phone. I asked if i can buy the battery from them and ask other repair shops to install it and they declined.

so apple can't fix iphones is basically what they are telling me. I am not taking advantage of their warranty or anything, If the phone can be assembled back without the so-called "tube" i am sure people can take it apart and reinstall it.

What should i do guys? Is it really the final nails to my iPhone coffin?