The Mac Mini is a great substitute for the old Mac Pro is they really want user serviceability. You can configure it up to a 3.2GHz i7 with 64G RAM and 2TB SSD for around that $2999 price (I'm in Australia so I can't check, but it's AUD$3500 which is less than US$2999).
The (non-Pro) iMac is a great substitute if they don't want user serviceability.
The problem with the mini and iMac (both varieties) is the GPU, which is too weak for many. External GPU via TB3 is a cool idea but it’s bottlenecked compared to raw PCI-E, which isn’t ideal.
I personally don’t even care if it’s AMD instead of Nvidia, I just want stronger card options. The fact that the GPU in the top config iMac Pro is just on par with my now-old 980Ti really sours it.
Note that I’m saying this as a long time Mac user and iOS dev using a hackintosh tower alongside a couple of MacBooks.
The (non-Pro) iMac is a great substitute if they don't want user serviceability.