Unless you plan to do some gaming, I wouldn’t upgrade just for the heck of it. The 3.7 GHz i5 is only about 10% faster than the 3.0 GHz i5 in benchmarks. Extra cores or threads have a better chance of keeping a computer feeling snappy 5-10 years from now, but unless you step way up to the i9, which is probably overkill for your uses, the i5s all have the same number of cores and threads and just slightly different clocks. The faster clocks plus the faster 580X GPU would be worth it for gaming, but for general purpose computing... web browsing, spreadsheets, watching videos, etc, you won’t notice the difference.
As to Zandros’ point about the lack of T2 being a good cutoff point for dropping macOS support, that’s probably true, but it’s not something you should worry about. I’m not sure if this is an official or unofficial policy, but Apple provides major macOS updates / support for a minimum of 7 years after a line of Macs is discontinued, plus 2 more years of security only updates. So even if Apple discontinued the current iMac 5K today for a redesigned model, you would still get major macOS updates until AT LEAST 2026, and security updates until AT LEAST 2028.
Also, to Zandros’ other points... if a redesigned iMac was released this fall or even in the spring, it would likely still be using “a CPU architecture from 2015” and “a GPU architecture from 2016.” Apple is using Intel’s latest chips, and I think the Vega 48 GPU is the newest architecture AMD has in the midrange price / performance segment. When Apple redesigned the MacBook Pros in 2016, they bumped the prices by 20%, so if there is indeed a new iMac coming soon, I expect you’ll be paying at least 20% more for the same specs. They’ll probably add smaller bezels, FaceID, always listening Siri, and T2, but I wouldn’t expect any life changing features.