Many nodes on the same HDD

The big node with RAID setup in such conditions will wear out all disks in the array and will take not less time to fill up, but it may be almost empty during this time in compare with size of the one HDD, so why wear out all of them?

Unfortunately it’s true for RAID5 too, with todays disks the array could be lost during rebuild after the one disk failure because of bitrot. With 16 disks it’s almost 100% probability:

In case of separate disks you could lose 1/16 of the common data (it’s distributed between them), and not all.
However RAID setup is different from running several nodes on one HDD.