Also crucially, due to Apple's spat with NVIDIA, the Mac Pro doesn't support CUDA. This means software has to be modified to use Metal Compute to support the Mac Pro.
If you make pro software Apple just let off a huge signal that the future of the Mac Pro is at best uncertain. So maybe hold off on that Mac support for the next two years.
Apple is not going to make an ARM Xeon. The resulting computer would be so expensive after you amortise all the R&D to create a single workstation class CPU for it, that nobody would be able to afford it. All the Pros who bought Mac Pro got played hard.
Anything that isn't a Hollywood studio will have to use an ARM iMac.
Enthousiasts and semi-pro's that want a powerful, affordable and extensible Mac with state of the art discrete GPU's can probably get lost, as is the case right now.
Then again, Apple is now only bound by their own operations, so who knows what they have planned.
Now, it turns out that Apple wants to complete a transition in 2 years... they are not keeping x86 around for the Mac Pro longer than that. It's not credible that Apple can just scale up their CPU into a workstation part. So if you are making pro software then you have to come to the inescapable conclusion that Apple is, in fact, not serious about the Pro market, and it's probably best to avoid expending further development resources on the Mac.
Not much. If you bought that machine you didn't care about it much as you bought older hardware at a premium price-point. You bought it to use it right now without fussing and have accepted it's obsolescence in 2-4 years, as is quite normal for studios.
If you bought it as an IT enthousiast, well..why would you even do that?
I don't see Apple coming out with an ARM Mac Pro within 3 years anyway. Why would they do that? No upside for them, that market has to be won back first. Slowly start with laptops and iMac, focusing on consumers, get the OS in shape and third-party vendors accustomed to the platform first.
iPad-like performance is already fast enough for almost all consumers, and I'm sure Apple doesn't want to break with Intel on everything right away.