I don't see why ZFS wouldn't have gone over equally flawlessly. None of the features that make ZFS special were in HFS(+), so conversion wouldn't be too hard. T

I don't see why ZFS wouldn't have gone over equally flawlessly. None of the features that make ZFS special were in HFS(+), so conversion wouldn't be too hard. The only challenge would be maintaining the legacy compression algorithms, but ZFS is configurable enough that Apple could've added their custom compression to it quite easily. There are probably good reasons for Apple to reinvent ZFS as APFS a decade later, but none of them technical. I also wouldn't call the rollout of APFS flawless, per se. It's still a terrible fit for (external) hard drives and their own products don't auto convert to APFS in some cases. There was also plenty of breakage when case-sensitivity flipped on people and software, but as far as I can tell Apple just never bothered to address that.