I am reading this thread and hoping someone eventually can make a 600 mm Dagor or Protar lens; I would certainly be a customer. It may be a complex undertaking, but I am sure that it could be done with the right resources.
It is interesting that Dave B. used an analogy of the auto industry to suggest you could not enter a field without massive resources and organization, but an entrepreneur in California has brought to market the Tesla electric car in the past two years, which is revolutionary. Admittedly, it was a very wealthy person, but it just shows what passion and commitment can do (and assistance from an established car company in England-Lotus). I still am amazed that the big 3 car companies are so slow to innovate and bring new products to market, wheras dreamers with passion can!