Still rock-solid office apps, and running multiple services/media servers, light video and audio editing. Yes, the problem is lack of 4K support. But unless we're talking about the cMP, or recent TB3 models, you would never be able to upgrade the video performance. So there is no Goldilocks Mac mini after all....
Not everyone who buys computers wants or expects to upgrade machines, and none of the major hardware vendors are heading in that direction, anyways. Why would you expect Apple to be any different?