That means that instead of hopelessly compromising thermals and power supply with some power hungry, yet limited GPU (because of the small form factor) you simply buy the CPU and memory that you need. It simplifies the job of keeping it cool.
CPUs are evolving a lot slower these days. This one should last you many years before it becomes a problem. Having upgradeable memory all the way to 64GB means that too is not going to be a problem any time soon. It means that a mac mini should have a serviceable life of 3-5 years or more if you are less of a power user.
Additionally, you plugin an eGPU of your choice and additional storage via USB-C. Better, when improved eGPUs become available, you can sell your old one and buy a new one without having to tear the machine apart. They also don't overtax and compromise your power supply or cooling.
I currently have an imac 5K that is nearly 5 years old now. I maxed it out at the time with all the bells and whistles and it has served me well. So, money well spent despite the shocking price initially.
I'd totally consider spending 3-4K on a setup with a mac mini, decent eGPU + monitor, and external ssd storage (I actually have a Samsung 2TB T5 already). The new imac pro would cost more and deliver less value. What I like about this setup is that it is completely modular and I can replace individual components without having to worry breaking the other components.
I imagine the mac pro next year will also emphasize expansion and upgrades through USB-C rather than internals. Basically the old model without dedicated GPU and upgradable Xeon CPUs + Memory would be exactly the right product right now. Egpus can be replaced easily and with a solid base configuration, a pro machine should have a long productive life.