I agree on the top level that a desktop computer in 2018 is actually probably more niche and pro focused than laptops, which have become the default computers for people.
But Apple has the iMac Pro and the upcoming Mac Pro update. So, what would a pro-focused Mac Mini be? It would need the reliability of a desktop machine, something the laptops have struggled with.
I guess I could see this being useful for someone who wants external displays, but doesn't need that much power.