I would think most standard consumers are not using HDDs at all.

The pool is about 20 usable TB.

Thats exactly what I wanted to say, yes :D.

I have 2*12TB whitelabel WD drives (harvested from external drives but Datacenter drives accourding to the SN) and one 16 TB Toshiba white-label (purchased directly also meant for datacenters) in a raidz1.

How full is your pool? I have about 2/3rds full which impacts scrubbing I think. I also frequently access the pool which delays scrubbing.

I am not questioning the need for more storage but the need dor more storage without increased speeds.

And I’ve only ever had WD hard drives and sandisk flash drives die on me

Maybe it’s confirmation bias but almost all memory that failed on me has been sandisk-flash storage. Zhe only exception being a corsair ssd which failed after 3 yrs as the main laptop drive + another 3 as a server boot and log-drive.

What is the usecase for drives that large?

I ‘only’ have 12Tb drives and yet my zfs-pool already needs ~two weeks to scrub it all. With something like this it would literally not be done before the next scheduled scrub.