Really doubt they will go down that route. The last Mac Pro had dual GPU in every configuration and it seemed to be a bad decision. I think Apple betted on multi GPU being the future, but it just didn't turn out that way.
For gaming, yes. For production workloads that leverage CUDA and its AMD equivalent, not remotely. Particularly anything scientific research related, deep/machine learning, etc, multi-GPU is alive and well and is definitely the future. Remembet that not everyone who buys a “Pro” machine is a graphics professional (where Adobe still has trouble getting Premiere to use more than six cores at all, and the other CS apps are even less MP-aware...).