The board is already cost-prohibitive for most people! They probably lose money on these alone -- the tape-out for a 28nm ASIC is millions alone, they're never getting that back off niche development kits.

Plus, given it has a custom ASIC and rather newer components like DDR4 (DDR4 controllers aren't exactly trivial on their own) and FMC (not simple, high cost), I'm having a hard time feeling sympathetic for this particular complaint, especially considering bottom-of-the-barrel boards like the OrangePi & networking competitors like MacchiatoBin can stack in multiple SATA ports on devices in the $50 to $350 range.

I mean, the board is already $1k, and I doubt they're going as far as home grown USB/UART/JTAG chips -- probably FTDI chips, so some of the open hardware claim is a bit fluff in practice, I'm guessing (I don't think this is a huge deal, but many people do). You'd probably use an off the shelf SATA controller & chip, it's not like you really get a lot from rolling your own.

Then again, I've never taped out a board with SATA, so what do I know? But I find it hard to believe the difficulty/cost of acquiring the chip/controller, or integrating it, is a limiting factor in a run like this. Unless they actually planned on rolling it themselves, and I don't know why they would. I'm honestly guessing they're just leaving it to expansions for whatever reason, but we'll see.