There are lots of people that don't want a ATX-sized machine. Let's face it, even mini-ITX machines that are capable of high-end graphics are not very small compared to a NUC/BRIX, and they're still full of 90% air. My sincere hope is that the emergence of external graphics will foster some innovation in the form-factor.
Without the requirement to fit into a graphics card slot and draw power from 12V PCIe connectors, it should be possible to make an external graphics box that is much smaller. You can imagine that with an external power supply brick, an external graphics box wouldn't need to be much bigger than a NUC/BRIX.
Of course, before any company makes that kind of investment, they will need to know there's a market, and that will likely be determined by how well these ATX-card-in-a-box solutions sell.