HDD docks can be flaky, certainly for 24/7 use, and they limit your options. I don't know whether you're using any kind of pooling or parity over USB, but I sure wouldn't. External HDDs can be useful, of course. If all you want is a simple, low-activity storage volume for your desktop, then an external HDD will work just fine. They're also excellent for offline backups; I have several of those. But external drives aren't great otherwise. You don't have to take my word for it: HDD manufacturers more or less acknowledge the flakiness of external HDDs via their warranty policy.
Expansion cards for SATA? Likewise flaky. Lot of pitfalls in that market, often having to do with
the stealth inclusion of port multipliers. Your most reliable option if you need lots of extra SATA ports is an enterprise grade HBA, which can be bought used for a reasonable price from a reputable vendor on Ebay, but you do have to do your homework (e.g. make sure the product is in IT mode), and the total cost for this set up, once you factor in breakout cables, and possibly supplemental cooling (because HBAs can run hot), isn't trivial.
It is preferable to have some amount of onboard SATA to work with before you have to resort to the aforementioned methods. Maybe not a ton, but some. I was happy with six as the standard. The Z890's "up to eight" sounds promising too. As
@TheLostSwede says, it isn't a requirement to include eight ports, but why not have a couple of models that do?
I don't think your ODD analogy holds up. Maybe some day HDDs will be comparably superfluous, but today the comparison isn't even close. And we also still have SATA SSDs, which retain a fair bit of utility in large part precisely because they can be plugged in via SATA instead of taking up an M.2/PCIe slot. In any case, it's always struck me as bizarre when DIY PC enthusiasts argue for fewer options.