The original (2G)wasn't available in my country iirc, & I had my own phone/MP3 player at the time - Siemens SX1...what a fun/irritating device that was. 
My plan was to not upgrade every year, so after buying the 3GS (outright), the 4/4S were never in the equation, besides, I couldn't really afford to upgrade every year at the time.
SE wasn't my size - had the 5 before it & wanted to go bigger, so the 6S became my next purchase.
The 7 series was a dead series for me, for two reasons - not enough of an upgrade over the 6S & also, I knew the X was coming the following year - yet another screen size boost I was eager for.
The 8 series was never in contention...all about the X last year.
So all up, i've bought into 4 series of iPhones, skipping others mainly due to not enough of a technological reason to upgrade (for my needs), with financial reasons a distant second.
My plan was to not upgrade every year, so after buying the 3GS (outright), the 4/4S were never in the equation, besides, I couldn't really afford to upgrade every year at the time.
SE wasn't my size - had the 5 before it & wanted to go bigger, so the 6S became my next purchase.
The 7 series was a dead series for me, for two reasons - not enough of an upgrade over the 6S & also, I knew the X was coming the following year - yet another screen size boost I was eager for.
The 8 series was never in contention...all about the X last year.
So all up, i've bought into 4 series of iPhones, skipping others mainly due to not enough of a technological reason to upgrade (for my needs), with financial reasons a distant second.