HDDs - are never one time costs. Do datacenters also offer ordering and replacing HDDs?

With 30PB it's likely they will simply let capacity fall as drives fail.

They apparently have zero need for redundancy in their use case, and the failure rate won't be high enough to take out a significant percentage of their capacity.

They offer replacing, yes, but normally expect you to order the new one. (Usually covered by a warranty, sent next business day.)