But with an eGPU you have the option to upgrade and the option to use a separate display.
If you buy an iMac you cannot upgrade your graphics whatsoever, you're stuck as-configured. Further, if you intend to use an eGPU with an iMac, you MUST get an external display, since target display mode doesn't exist on the new iMacs.
The Asus STRIX RX Vega 64 is $590, and let's assume a full-tilt i7 32GB + 512GB SSD Mac Mini is $1200 (hopefully that's high, but knowing apple I doubt it.). The required box for that is $400 (Sonnet 650W).
Assuming you have a monitor already, total cost for a VERY capable Mac mini is $2190.
Contrast with a full kit iMac, where you don't even get close to Vega 64 graphics performance, you'd be paying $2600 ($410 difference)
$410 can get you a 4K monitor (or a couple 1080p monitors), and you'd still be exceeding any iMac performance, unless you got an iMac Pro.
I hope they're not stupid with this though. I think they'd gain a lot more money on updated Mac minis and selling GPU enclosures than from snubbing the Mac mini and pushing harder on iMac/Pros.