Until he passed in 2018, a buddy was using an original iPhone. He refused to upgrade it even once. It seemed to work fine for his use.
My original iPhone (2007) was handed down through my family, with its original battery which was still adequately working for eight years, until it was finally retired after being used as the equivalent of an iPod touch (no cellular phone connection - WIFI connection only for kids games) by my then five year old granddaughter. At that time that original battery would still take an 80% charge which would last about ten hours.