Having been responsible for several multi-petabyte storage projects, my experience is that TCO over a 20-year archive lifespan (LTO's design spec is 30, but boundary-pushing is not a good idea in the long-term storage game) is much lower for tape. Indeed you can't generally get reliable parts and support for disk shelves beyond about seven years, which means a full-solution refresh at least twice as often as with tape.

Most of the time the right answer is most of the storage in tape, with a couple of petabytes of disk in front of it to deal with hot data. Even in that circumstance you have to carefully manage the disk storage and filesystem or you wind up with client devices too new to access the supported fs version, another problem that basically does not exist with tape.

That's a good point - there is no such thing as a tape-only data storage system - you'll need RAM, flash, and HDDs to help ingest large quantities of data so that tape drives can have data ready to write.