It’s going to be a tough decision to sink that much money into a machine when a switch to arm could be coming the next year - and I like my shiny new toys at quite rapid intervals 😕
I'm not very worried about a potential switch to arm in the near future for pro devices. I could definitely see it for the Macbook or Mac Mini (if an update ever comes), as those devices are not centered on top performance. However, for pro devices it can't be that easy to fabricate chips to compete with what Intel has/will be putting out in the near future at the top end. Intel has been doing this for decades, and as is currently being seen with the move from 14nm to 10nm, it's not an easy business.
Furthermore, if you look at Apple's Marketing/Keynotes in the past, they love to show the slides that spell out how much % faster current models are compared to the previous ones. I can't see them taking a potentially major step back in performance for pros just to own that part of the device build. Apple could definitely get there, but I find it hard to believe they'll catch up in as soon as a few years.
Additionally, the massively increased size of the mac app ecosystem at this point would require the transition to take place over a longer timeline than the last time Apple did this. Devs would need to rewrite their apps for the new architecture, and there's no better way to motivate pros to not buy your computers if the apps they rely on to make a living are no longer compatible or unstable at best.
I'm waiting until 2019 (Ice Lake) or 2020 (hopeful redesign to see if the touchbar has a future or will just be a waste on older machines as they drop it), but even then I would be confident I'd get 3-5 years out of the machine unless we hear something concrete. And 3-5 years is a good enough timeframe to make it a worthwhile investment until assessing the state of things for the next hardware purchase after that.
And even if Mac OS support goes away sooner than expected, you still have excellent hardware to run Linux on.
