It would also require Apple to develop, manufacture and stock different "base" modules with system boards using consumer i5 / i7 CPUs and those using Xeon CPUs. You would then also have multiple BTO trees dependent on those architectures and what they support in terms of RAM type and capacity, PCI lanes and external connectivity (both with other Apple modules and third-party devices).
I do not see Apple wanting to support that level of complexity internally nor expose it to their customer base externally.
Everything involves trade-offs, and I suspect that the one that Apple finally re-awoke to is that "Hardware Specs Matter", with the perspective that working a bit more upfront on a flexible base design has payoffs down the lifecycle when it comes to the later generation derivatives of that base design, as opposed to a "One Off" design that has to be figuratively 'entirely' thrown away in order to conduct any update.
For example, even the 6,1 Mac Pro had different CPUs that could be plugged into its motherboard's CPU socket, and one of the (few) things they did to try to refresh the product line was to replace a 4-core CPU with the 6-core. In essence, this is because the design's motherboard was "modular" because of the CPU socket.
Granted, there's more to the design details than merely the CPU socket .. there's thermal, issues of bottlenecking, etc, but in the end, they sold the 6,1 Mac Pro with a variety of CPUs and then were able to 'refresh' the tcMP (however mildly) a few years later to a (slightly) different CPU selection group.
RAM is a similar 'modular' beast when there's a slot for it instead of being soldered on. Ditto for HDDs/SSDs.
And sure, there's going to be OS & Software support required for changes such as a newer (or previously too expensive) CPUs, but these can be planned & scheduled ... and in the big "enterprise" level picture, its cheaper to add & test some new code than it is to design, build & test a new motherboard
in addition to the same OS code revisions.
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In any event, as more & more time slips buy without a shipping product, the more that I'm convinced that much of Apple's lethargy here is that they're trying to figure out how to be 'modular' from their own manufacturing flexibility perspective while keeping the changes cheap (for them) but while not maximizing their use of existing industry standards (such as PCIe), because doing so results in an erosion of profits because of losses to DIY'ers.
And sure, we can go point to alleged statistics that say the likes of "90%" (or whatever) of Mac users never revise their hardware, that's a pretty darn nebulous claim when you start to really look at it. For example, since over 80% of Mac customers today are using laptops which can't be changed, while the statistic is pedantically correct that "only 10% of all users" revise hardware, its ignoring that 80% of them can't make changes even if they wanted to. As such, the 10% is all contained within the 20% who can make changes ... and 10%/20% = half of them. Suddenly, the metric isn't so dismissive anymore, isn't it?
