How many people are buying these 128GB discs? Probably not a whole lot and that is the reason why nobody’s going to make the investment to create bigger discs.
If you want huge local offline storage, the solution is tapes.
But it is already known that magnetic tape is much less reliable in long term storage. The author is looking for long-term reliability, not just huge local offline storage.
It is not known what the durability is for an optical disc format that does not exist. There is no optical disc format that stores the required amount of data.
LTO tapes do store the required amount of data and if you want them to last extremely long you should make new copies every 10 years or so. Just like you should for optical media. Sure, some discs might last 50 years. But some others won’t be readable after 15.
If you want huge local offline storage, the solution is tapes.