I think the iMac Pro isn't doing as Apple expected, in about 2 weeks from launch is now available at most locations for in store purchase, either Apple fine-tuned its distribution/mfr chains to the extreme or is not selling as good and then there are sudden inventory... and even its BTO ship times are now 1-3 business days...
Overall, I suspect that most of what we saw was Apple manipulating customer expectations - - when you don't have a hard delivery date target to hit, you can delay until inventories are healthy, then open the doors with relatively conservative delivery dates ... watch the demand, and then adjust. All that the drop in BTO times really means is that they did a good job anticipating what customers are going to order and had adequate reserves already fabbed to ship.
The good: Apple will understand that Pros don't buy sealed machines, and the iMac Pro will never be a substitute for the Mac Pro.
One can hope that Apple will grok customers who don't buy sealed machines .. but I'm not being optimistic.
In any event, Apple did quite poorly with the iMac Pro launch. Digital Lloyd has already ripped Apple for the iMac Pro, for two very basic reasons:
1. "Form OVER Function" (again), as illustrated by de-rated chips because of thermal.
2. Lack of Basic Business Sense (& professional courtesy): they withheld the details a business needs in order to make a buy decision, which compounded with their nearly-end-of-tax-year rollout (and ship dates) which made it impractical to buy & deploy by 12/31 in order to use the {USA} 179 tax deduction.
And FYI, I believe I read that the 179 just got killed for computers for small businesses in the tax bill ...?
And these are some of the factors which Apple will use to rationalize to themselves which are other than customer-based hardware modularity...
...particularly since I think that
Zarniwoop hit it on the head when he showed how Apple's language on modularity is that it really is about what Apple wants for turning around faster updates to sell to customers - - which is not modularity that can be DIY'ed by customers because then Apple doesn't get their 30% profit/cut on that component.