Frank Azor cites low interest in gaming subnotebooks like the Alienware 13 as a big reason for skipping additional GPU options for the XPS 13 family. A few competing 13-inch Ultrabooks like the HP Envy 13, Huawei MateBook X Pro, and Razer Blade Stealth all offer optional GeForce MX150 graphics for superior graphics performance over the XPS 13.
https://www.notebookcheck.net/Dell-has-no-current-plans-for-another-XPS-13-with-Intel-Iris-Pro-graphics.386476.0.html
Well, Intel is going from 24EUs to 64EUs in Ice Lake U and adding support for the much faster LPDDR4 (Iris Plus has 48EUs, though it has a much faster 128MB eDRAM sideport memory).
The Iris GPUs were mostly hamstrung by Intel's insistence that they use different packages than the regular chips. This meant the Iris SKUs always needed different motherboards and were never a drop in replacement.
Maybe Dell wants Alienware m13 to be their compact multimedia laptop.
Whiskey Lake brings 16 PCIe lanes and still no dGPU. Finish them, AMD.