So there's no more hard drive, only a PCIE-based NVMe SSD. The largest consumer hard drive is 12 TB now, but there's no longer an Apple machine on warranty that will hold it.
Well they do have those Thunderbolt enabled external enclosures. Since TB is backed on PCIe I believe you'd be able to get max performance on a 12 TB HDD.
If you want more storage, an external HDD gives you all that without speed comprise.
HDD is now speed limited, its capacity has continue to grow but it is actually getting slower. By mid to late 2018 once all the NAND and DRAM Fab are online we should finally see the decline in pricing, and I expect 2019 we will see new history low of NAND and DRAM price / GB.
So the HDD market is now really left with external storage, and Server market.