I half agree with this, I have two iMacs that predate retina displays that still run fine under load.It's probably not a good idea to keep any computer for 7 years, unless you're just watching movies on it. I have the first 5K iMac that ever came out, and I can't even watch Youtube on it anymore. The fan just spools up after a minute and the machine goes to sleep. As a first-generation product, it didn't age well. And anything static burns into the screen after hours, and it stays burned in for days. I can assure you the latest iMac is nothing like that. I haven't even heard the fan yet, and I do edit 4K video and run VMs.
Admittedly one had 2 (maybe 3?) screen replacements (free, thanks apple!) and a DIY fan replacement (Sounds like yours might need similar!), and the other has image retention issues sometimes, the hinge spring exploded so I had to buy a 3d printed clasp, and the HDD part of the fusion drive burnt out so I have to run it from an external thunderbolt SSD..) but they’re both daily drivers, mostly used for photography/image editing. Core performance on both is great, its just everything else that’s cumulatively wonky. 😂 (I keep impeccable care of my machines and they’re kept in a clean environment..)
As much as I like the iMac form factor I wouldnt trust it to stay nice, so I’d not get a modern one either now. They, by design accumulate dust inside with no way to clean it out. Until recently Apple clearly couldn’t even reliably seal the screen and stop dust from depositing between panel layers, despite no other manufactures having this problem. I have a love/hate relationship with these machines.
i‘d probably get a fastish mini now for peace of mind. 😕