I am very happy with my 2017 iMac (i7, 1TB SSD) which has everything I need to stay with it for very long time. The 2017 iMac replaced my 2014 5K iMac that only had a 256GB SSD, no USB-C and no P3 display and a much weaker GPU. Before that I had a 2008 24“ iMac which I used for 7 years with an upgraded 512GB SSD. The 2017 iMac is something similar regarding the staying power.
I won’t upgrade the 2017 iMac with any kind of Intel based iMac in the near future! The only upgrade that might make sense in some years is a ARM based IMac with a completely new design, a bigger bezel-less display and a new cooling concept.