Well the CPU was $50 cheaper so that offset the ECC premium. I cared about ECC much more than the 166 MHz difference or whatever it was. For people buying an i7 (this was before the i9 I believe) + 16GB ram in 2015, $100 is a pretty small premium (less than 10%). Decent mid range CPUs were $300 (gtx 1070). There were lower end Xeons equivalent to the i5 as well (lacking SMT/hyperthreading).