"The pricing for each TB after 2TB is $150/TB"... no it isn't:
Upps, my bad. Thanks for the correction.
However one could also argue, that most users will use a great deal of their storage for content storage, rather than as a scratch drive for editing. So do they need the whole thing to work at 8GB/s speeds; no.
But Apple don't do a cheaper half and half storage machine: say 2TB superfast 8GB/s speeds + 6TB slower SATA or NVMe speeds. Clearly they can't separate the two functions of storage out...
External storage is the new hybrid drive isn't it? I was at the Apple Store once a couple of years ago and person after person came in with an external SSD velcroed to the back of their displays. It'd be less convenient, but I could live like that if necessary.
Plus, I like the idea of building up an arsenal of fast external storage drives that'll continue to be useful for many years to come. I have around half a dozen high capacity HDDs that are still living out productive retirement years as deep archives.