I mean, I'm not the type of person to upgrade a Mac that often. I went with the i9 because I'm planning on this lasting 6-8 years for me (well, unless I get screwed over by Apple dropping support for Intel in macOS after switching to Apple chips). I went with the Vega 48 so I could do some moderate gaming in Windows and for some GPU acceleration in pro apps (especially as apps are more optimized for that in the future) but I don't think it's worth it to upgrade for the small performance increase, especially since eGPUs are a thing (I could see myself getting one after a while). I think it would only be worth it for the i9 if you're utilizing a heavy multi-core workflow or if it's too noisy for you if doing audio work (though the iMac Pro is even quieter). If I had upgraded in 2017 I'd probably wait until at least 2020/2021 to see what Apple does with their chips, although I am kinda wary of buying a first gen Apple CPU Mac given the issues with the T2 chip which kinda factored into me upgrading now.
Totally fair point, I mean going from 4 cores to 8 cores is a big deal and also it seems like the Vega 48 is a decent upgrade but also no T2 baby wooo!!!! This is probably the last iMac without a T2 chip so another reason why I was thinking of selling/upgrading but your most likely correct in its not worth it.