iFixit has found that while the new 21.5-inch iMac is not easy to get inside, the RAM, 2.5-inch drive, and CPU are all technically user-replaceable.
https://www.notebookcheck.net/New-21-5-inch-iMac-has-upgradable-CPU-RAM-and-2-5-inch-drive.226548.0.html
the CPU wouldn't outdate even i still using i5 760. the graphic card are the major part we need to upgrade.
1.GPU , 2 RAM , 3 SSD , 4 M.2 etc.. the last will be CPU.