I think the big, looming problem with 70mm is the cassettes at this point. Mags will last more-or-less indefinitely, but there is an aging supply of 70mm cassettes on the market. I've bought an amply amount on the Bay, but long-term, if it dries up, it could be a problem.
Sure you can replace the felt, but the METAL can be a problem. Paint flakes off of it when you tape around the ends for fear of light leaks from warped metal connections, and this can cause dust and dirt. Plus paint fleck makes for an even looser fit and there is no protection from further metal ware.
Kodak used to make them, but no longer, and we all know how Kodak is about sharing old technology with other companies. Linhof also used to maake them. If they are still in business as the same company there might be a chance of getting a batch manufactured. After all, they are just hunks of metal.
Even better, though less likely, would be to make them from plastic, like with the screw-top caps. IDK with the tar-drop top shape if it would be possible to engineer a design with a screw top. And a new spec. would be costly.
Does anyone know if there were/are any other manufacturers of the cassettes?
Maybe it would be possible to get an aerial photography company, as they still use 70mm, to go in with individual purchasers on a cassette order.
As to wanting Hp5 over Delta, why haven't you been getting Tri-X all this time? Whereas with color films, it is easy to see a different, B&W films of the same type are far more subtle and difficult to tell apart, unless it is physically poor like some of Kodak's papers used to be. Ilford beat them hands-down there.