Bill Gates himself said that the goal for lifespan of Windows computers was 3 years, with heavy users upgrading every 18 months. This was a while ago, and one thing that has happened in between is the Great Intel CPU Stagnation. Years ago, the expectation was that CPU power would double every 18 months to 2 years (Moore's Law). Now, that's gotten close to 10 years. When you add in Apple's being two CPU generations behind, it IS 10 years on the iMac). I looked this up on EveryMac, using the single core Geekbench 4 score for various generations of the top 27" iMac from 2009 (first quad core i7) to the present. Multi-core scores actually progress a little less rapidly (although a 2019 iMac that has more than 4 cores will see a huge boost in multi-core score, possibly coupled with a decline in single-core performance).
Mid 2009 - 2536
Late 2011 - 3506
Late 2014 - 4740
Late 2015 - 5275
Late 2017 (current) - 5581
Only a little more than double the CPU performance (about double +10%) over a decade...
The new iMac when it comes will probably be only slightly faster in these single core scores (maybe slower), but it'll have a huge multi-core boost. For reference, an 8-core iMac Pro is about 10% slower than the top 2017 iMac in single-core performance (5040 vs 5581), but 40% faster in multi-core performance (30893 vs 18914) due to the higher core count.
No iMac Pro is as fast as the 2017 iMac on one core (the fastest is the 10-core at 5302), but they all have very high multi-core Geekbench scores, right up to the 18-core at 47221.
The 2019 iMac will probably top out right around 5800 on single-core Geekbench, but it will have a multi-core score above 30,000, maybe as high as 35,000. The majority of PC Geekbench 4 scores (excluding heavily overclocked scores at the top) range from about 5800-6200 single-core, with multi-core scores around 33,000-36,000. I'm guessing the iMac will be towards the bottom of these range, both because of cooling issues and because many PC motherboards (and system builders) apply a slight default overclock, especially on performance boards that are likely to host a K-series chip. Apple won't do that.