First, try to edit a large project with 4K video and you'll soon find out. Unless you transcode for hours, the machine will be on its knees, especially with multicam stuff.
Second, computing power is not a binary "down to its knees" / "manages to do it".
I always could use a faster machine (I can afford and upgrade eventually) as a pro, even when my current machine is not "to its knees"
If it does its rendering in, say, 4 hours, it's always welcome to be able to do it in 3 hours or 2 hours. Those are hours off of my time.
If PC-using pros can do it with their GPUs/memory/etc, why shouldn't Mac using pros be able to keep up? We do compete for the same jobs, you know.
Now add video FX rendering (e.g. after effects) with many nodes and layers, 3D rendering (which already can take days for a large project), and so on.
And lastly, the iMac with eGPU is still a hassle (no upgrade internal GPU, extra costs for the eGPU cage and cables, occasional eGPU-related glitches, etc). And it's still non memory upgradable, non CPU upgradable, non SSDs upgradable, and with a screen that can't be used as a standalone screen when you get past that machine.
So, no an "$5K for an iMac Pro" is not a replacement for an upgradable rig, that can take non-Apple-marked-up upgrades, and doesn't have to come with its own monitor.