What does a 4TB SSD upgrade in a PC cost?
If the drive uses old fashioned SATA-6000 interface, around $1,050 for a 4TB (Used to be closer to $1,500). You could do that in a Dell XPS since 2015. I've used these laptops - 500MB/second may sound slow, but IMO, you get almost all the benefits of Apple's 2GB/second drives - you can barely notice the difference. (But the difference from a spinning hard drive is extremely noticeable!)
The big choice not being offered to customers is this:
Is the difference you feel going from 500MB/s to 2,000MB/s worth $3,000 to you?
At the next Tier, if you got a Windows Laptop with NVMe drives, it would cost you about $2,000 for a 4TB drive (which admittedly I haven't seen offered yet) - that's about half what Apple charges, but again, the drive would run slower, closer to 1,000MB/sec. So again, the question here is "does the feel you get going from 1GB/s to 2 GB/s worth $2,000 to you?
Honestly, I really think Apple is mainly trying not to undercut their older models. Maybe when that inventory clears out they'll be willing to lower prices...
UPDATE: For a concrete comparison, the latest DELL XPS 15.6" has almost identical specs to the 2018 Macbook Pro: Same CPU, same 32GB DDR4, similar weight, a higher resolution screen, far more ports, etc, etc. Their model with a 1TB SSD costs $2,900. Could not find prices on their upgrades, or if they are using NVMe.