You contradict yourself with your opener there. If their in house processors fail to show year over year improvement in future and other avenues are providing considerable performance increases, their users will drop them.
I'm not contradicting myself. And users won't drop Apple; they're quite loyal and many will soldier on even with NO hardware upgrades at all, as they did when Mac Pro wasn't updated for (IIRC) 3 straight years before introduction of the Trashcan, and then was not updated for another 5 years or whatever it was before current Mac Pro came out on the market last year. Certain other Mac product lines have also gone multiple years without upgrades at times (Mac Mini and Air especially), and people still stuck with their Macs, and with Apple.
With iOS and Mac hardware sharing the same basic hardware architecture, the risk of no upgrades at all for any major length of time is basically going to be zero. I'd say two years at most between major SoC overhauls; second-year upgrade may be a reheated first-year SoC, like current iPad Pro's A12Z.
And you're loco if you think any substantial amount of people upgrade their machines on a yearly cycle, pro users or no. That's not happening, because it's not even remotely cost efficient. And like I said, essentially no users buy Macs for the hardware performance. There's faster options out there for pretty much everything you can imagine. And this certainly includes the Mac Pro, whose best graphics option is a several-years-old GPU which was ho hum from day friggin' one! Unless of course you're doing digital video editing, in which case Apple's add-in accelerator board may be your best option on the market right now, who knows.
I wouldn't put it past Apple to consolidate their stack in a few years and terminate low volume products.
Technically, the entire Mac line is low volume, yet Apple still persists with it, and arguably spends more resources on it now than ever. So I wouldn't think that's any sort of issue going forwards.