Not a charity
So there are a handful of organisations that use supercomputeres that generate volumes of data which are logistically difficult to deal with on the prospective LTO6 and the question is what are they meant to do if the LTO roadmap stops at LTO6? Well, in a breaking piece of news, I hear that in a real shocker that the storage supplier market isn't driven by charity designed to meet every requirement. It is, believe it or not, driven by businesses that will seek a return on their investment. There may well be a handful of organisations in need of tape backup solutions which scale into the tens of PB without taking up warehouses full of cartridges. However, such a handful of organisations does not necessarily make up a viable market.
To put (say) a requirement for a gross 100PB into perspective (allowing for redundancy like multiple copies, disaster copies and son on) with 3TB LTO6, then that's about 33,000 cartridges. Which is a lot, but there are systems out there where individual tape libraries scale to approaching 10,000 cartridges. Most of those 33,000 cartridges will be duplicated offlined somewhere for archive and safe keeping. It's a bit of a logistical problem of course - archive copies have to be regularly refreshed as there simply isn't a long-term archive storage medium that can be trusted.
Of course it's very unlikely that data is processed direct from tape, apart from the initial slection. The days when data was sequentially processed direct from tape are probably gone for the vast majority of organisation - the data has to be brought online to disk first. There is the real problem - as many people struggling with very large data volumes can tell you, it's the transfer of data to and from tape that is often the biggest bottlneck. The throughput of an LTO4 tape is such that is considerably exceeds the throughput capability of any single disk (apart from SSDs). It even stretches the capability of many RAID controllers. Start running several LTO4s flat out at the same time, and it doesn't take many of those to saturate many medium sized storage arrays. Of course the supercomputer user will have all sorts of highly specialised parallelised and distributed storage systems over their cluster to handle the huge I/O rates. However, that's not a route open to most companies and consequently the market for huge tape drives with many hundreds of MBps each is very limited.
The problem with tape is often not the basic capacity, or even the sequential throughput, it's that issue of moving data between nearline and online storage media. Hence LTO is concentrating on capacity and not throughput.
One more calculation - 100PB per year is an average of about 3GB per second. If we assume that all data is accessed 10 times per year, that's 30GB per second or about 120 LTO6 drives. Double that up for 50% util and we get about 250 drives. If you are an organisation that has this much data, then 250 drives and libraries isn't an impossibly large bill - but coming up with the supercomputer that could deal with the implied peak bandwidth of > 120GBps (assuming moving between disk and tape) is going to be some engineering challenge.