Yeah, I feel you guys are forgetting that AMD has Infinity Fabric, meaning they can put (up to) 4 gpu's on a single PCB within a single die "theoretically" and eventually 8 when they hit below 7nm. Nvidia is working on something similar. mGPU is no longer going to be limited to multiple PCIe slots it will also be for single slots and PCIe4/5 will allow such possible upgrade path. With high enough bandwidth and low enough latency (mGPU) will have a slight latency increase like the Threadrippers, but nothing like SLI/Xfire or dual chip single PCB's.
Look on the bright side PCB's should get smaller, and we could see a 80% if not more increase in GPU Performance over it's single GPU chip and 20 to 30% over the current mGPU dual chip PCB, being on the same die instead of tied together through small wires with a lot of distance between, makes a lot of sense for the next step in GPU acceleration.