Quad GTX 1080Ti outperform a single VEGA 64 - true but I would expect that the majority of quad GTX cards setups are found in servers rather than single user work stations. There are many who have access to compute servers that make a quad GTX machine looks like a joke compute wise. I think Apples line up the last years has been aimed at OK work station performance connected with strong servers in the background doing the heavy lifting. This is not a particularly unique setup.
A single GTX1080Ti outperforms a Vega64.
doing ML Training in server farms is not viable/practical there are many factos as network banwidth configurations etc (while google is trying this concept in near future, and Amazon sells GP GPU server time ), server farms in ML are best suited for productions services as Siri, Alexa, not for R&D which requires a lot of flexibility and customization not trivial.
For my information: Are these 4-16 GPU used for iOS Development? Are these machines used as servers of workstations connected to servers?
Sadly currently no ones uses more than 2 GPUs in iOS development because is the max available configuration in the tcMP coz lazy Apple managers didn't care to update it.
In Android (and many others unrelated development) you may need all those GPUs (and even more if available) to train again and again an AI until you manage it to do what you want, then you train it for production, finally the AI is sent to production servers or the mobile devices where the inferences are quite less demanding and can be handled by simpler gpu or dedicated NN.
I am not apologising for anything - just drawing conclusions based on possible strategic scenario: 1. Largerst earner is iOS devices. 2. developers need hardware to develope iOS apps. 3. Current line is propably sufficient.
Your conclusions are Naive, no matter if current hardware its OK for General Purpose Apps, those developers on the edge competing with other platforms need hardware on the edge.
Current line is enough if nobody on past 4 years ever mentioned ML, AR, VR.