You make a very good point. If I was interested in buying the iMac Pro I would definitely get double of the RAM and storage. That's why the base configuration might not be selling that well. Pros need all the power they can get and since your are already paying for it the base config. is not enough for most needs.
This Micro Center deal isn’t an Apple price cut, it’s a single retailer’s loss-leader sale price. Micro Center is well known for this, they’ve been doing it for at least 5 years that I can remember.
The base config is plenty for many iMac Pro customers. Of course, certain workloads can use more cores/RAM/storage, but price still matters to buyers. If I had to guess I’d venture that 1/3 of iMac Pro sales will be the $4,999 base model, with another 1/3 coming in under $7,500. (A 10-core, 64GB, 2TB unit is $7,399.)
Specifically regarding the CPU config, I wouldn’t be at all surprised if Apple sold more of the 8-core than the 10-, 14- and 18-core combined.