When you think its time to upgrade your Mac, you should also consider ‘sidegrading’ to a Windows based machine (or other operating system).
The iMac is still a fantastic all in one package, but you have to ask yourself just how much value you are really getting out of it for the amount you spend.
Unless you have extremely processor intensive apps, any computer you purchased for 2–3 years (or more) before 2015 should still do the job. Now that there are computers available shipping with Windows 10 (shipped in 2015) performance is respectable. I upgraded a collection of Windows 8.1 machines to Windows 10 and performance is more than enough for business tasks (and more — I often use them for 3D rendering).
Apple is concentrating on the ‘middle’ to high ground of premium consumer products. The Pro Mac line is stagnant. The best entry level computer from Apple, the iMac Mini, was actually gimped and limited as compared to the previous version — meaning, the computers Apple will ship are relatively high margin packages for which you get a nice looking package with limited ability to upgrade.