sata has had hot swap for some time. We do not need file system level raid. We need block level raid Does not mac os not have mdraid?? (does not need to be boot disk)
Also don't force people into bios fake raid
APFS is not just a file system it subsumed both Core Storage and HFS+ . Fusion Drive is block level solution (the new APFS provisioned on will likely be one too). There is no "BIOS" that Apple like to incorporate into their solution at all. If trying to talk about software boot support nobody is being forced into that at all. If folks want to buy stuff and add it to their new Mac Pro later that would still be an option. Nor does it "have to be" a bootable solution. However, the Apple configurations that they sell just probably won't incorporate that. Nor will Apple likely sell any "real RAID card" in their store. It isn't a focus nor absolutely critical to the market as long as they provide slot for folks to plug in their "real RAID card", but those don't necessarily have to point at internal drives in Apple sleds.
SATA having hot swap is immaterial. Something up the stack from SATA has to make make that disk that disappearance doe not appear to the file system. The point is that Apple isn't going to be selling that. The Mac Pro 2006 with the custom RAID card is not where Apple is likely to go back to. The Mac Pro from 2007-2008-2009-2010 evolved to increasingly less aligned with internal redirection of the drive sleds to a "real RAID card". Again, it is highly unlikely that Apple is going to deviate from that evolutionary path.
also why not have e-sata??? it's free with the chipset and does not tie up the TB / USB bus. (other then the DMI bus)
Other than sunk costs in eSATA infrastructure, it is more performant than Thunderbolt v3 how? Like the MP 2013, the next Mac Pro likely will not put the TB bus behind the DMI bus. ( the iMac Pro doesn't either).
Given that the current x4 PCIe-v3 SSD are pretty close to saturating the DMI bus, that's probably a contributing reason why Apple isn't particularly keen on striped SATA solutions. If you put two x4 PCI-e SSDs on the PCH and actually use them at the same time then then can most definitely saturate the DMI bus. (e.g., Apple puts an empt M.2 slot on the PCH for an optional drive ).
If look at the iMac Pro block layout, they moved the 10GbE chipset off the PCH ( and DMI) path altogether; it hangs off the CPU lane block.
If Apple puts 4+ USB Type-A 3.1 gen 1 (or 2 if a new model takes even more than a year ) sockets on the next Mac Pro and provisions off the "already paid for" PCH then that will probably add to DMI bus traffic. And external SSD and T2 SSD in concurrent use DMI basically done. Apple is far more likely to use the 'free' high speed USB functionality of the PCH, than the SATA. That's another 5-10 Gbps.
if look in the general Windows PC workstation market the standard configurations are typically single boot HDDs which are not threat to the DMI bus saturation or perhaps some SATA SSD which are "throttled" back to SATA limits. (again no threat to DMI saturation). For the Mac Pro workstation market it is extremely likely going to be the exact opposite approach. The lowest configuration boot storage device is going to a blazingly fast SSD attached to the PCH which is relatively close to saturating the DMI bus.
At $3+ K priced systems Apple isn't going to be trying to sell these to folks who want to go affordably fast; it is performance for money class.
eSATA is more so a sunk cost pit that Apple could consider bumping the empty PCI-e standard slot count closer to 2-3 than perhaps just one. ( e.g., put a PCI-e switch on the link the 10GbE controller(s) is on and weave in a x4 PCI-e electrical , x8-16 physical slot)