My Windows machine is admittedly more modular than a mini+eGPU would be. I can pull the CPU out and put in a new one whenever I want! But over the course of 12 years and 3 computer builds, I've never done that once. By the time there's a noticeable CPU upgrade available I'd need a new motherboard to go with it.

So I think there's a big segment of the "modular" market that only really cares about having GPU options and upgradeable RAM.

It's not for everyone, but the people in between the high end Mac Mini (6-core i7 + thunderbolt GPU) and the low end Mac Pro (8-core Xeon W and internal expansion slots) are a small enough slice that Apple doesn't care.