This news really cuts short the lifespan of the new Mac Pro. Nobody will want those in a few years.
This happened with the PowerMac G5 back in the day too. The Intel switch was announced not long after the latest model G5 was announced, basically pissing off people who had just invested in new machines. Apple said they weren't leaving older machines behind just yet, but they did, very quickly. In less than a year nobody gave a **** about PowerPC anymore.
We're going to have the same thing happen if they transition to ARM. I'm pissed because this is unnecessary; the x86 architecture is fully capable. At least with PowerPC -> Intel they had a good reason for the switch.