I don't really understand what you mean by "re-spin". DDR5 won't come as surprise. CPU makers are taking part in memory development. DDR5 and appropriate CPUs have been developed together and can be launched together.
Yes, obviously all relevant parties have worked on their prototypes for years, in fact most standards are derived from prototypes, and are certainly not created in a vacuum.
But I don't get why so many people are fixated on DDR5, is there really a rush? CPU vendors will make the switch when it's needed and it's ready, not before. And based on the information I've seen, DDR5 doesn't look so good yet in terms of latency.
For mainstream users, dual channel DDR4 2666 MHz is plenty, and if you're a content creator you can always go for a quad-channel HEDT configuration. Memory bandwidth is primarily a bottleneck for heavy server loads, which is why upcoming Ice Lake-SP will feature 8 memory channels per socket. Higher memory bandwidth only really helps if a workload is bottlenecked, and over the past decade memory bandwidth have grown much faster than core speed, so generally you need heavy multithreading to be bottlenecked by memory bandwidth. As of now, DDR4 is supporting up to 3200 MHz, but I don't know if it can be expanded further.
Macs depend on x86 software ported from Windows. This is the reason Apple switched to Intel in the first place. They'll have to make sure every important piece of software is available for ARM. Most isn't.
Well, actually not.
The primary motivation was hardware. PPC promised to replace x86, but failed to do so. Apple needed an architecture capable of scaling from a low-power laptop to a high-end workstation, and only x86 could do that.
In terms of software, OS X have no relation to Windows or its ecosystem at all. The Darwin kernel is a mix of the mach microkernel and BSD Unix, and largely rely on the BSD ecosystem, APIs, compilers etc. while they are now increasingly drifting away from that and creating their own walled garden… Most of Apple's current software is already compatible with ARM, so that's not really a big concern.
But ARM is not close to performant enough to replace x86, and this wouldn't change anytime soon. ARM devices like iPhones and iPads rely heavily on specialized instructions to accelerate workloads, and anything else will perform like crap. If Apple switches completely to ARM it would either mean they focus solely on low-performance "web browsing devices" or a hornet's nest of software patchwork and specialized instructions to be "competitive".
But on an interesting note regarding AMD; Zen was supposed to be the steppingstone up to K12, the new big CPU architecture from AMD, based on ARM. Meanwhile K12 is MiA, and probably already obsolete compared to other ARM designs. So Zen 2, 3…5, is it the backup plan after the failure of K12? Is this why we don't hear anything about a Zen successor yet?