To echo Photo Engineer, I think the two facets most people underestimate is the quality control and non-scalability of costs. Nobody wants a product which changes with each roll or which has serious flaws in coating. Kodak itself tried to construct coating plants in China in the late 80's/early 90's which were never able to maintain the quality control film products demand. You have to remember that if your coating is off by 1/10 of a millimeter, it is out of alignment by perhaps 100 times. So, even if you have the formula, the set-up of the coating process and quality control will take as much money as the formula. Second, the reason a roll is under $5 is because they run it by the hundreds of meters at a time. Making 100 meters is not 1/5 the cost as making 500 meters, due to wasted material to spool the machine up to speed or slow it down prior to or after coating and quality errors. Finally, assuming you have the formula, have the equipment to replicate it, have properly calibrated machines and have run several hundred meters of film, now you have to have the supply chain to sell it. Would you trust brand XYZ Tri-X? How many people assume it is different and simply drop it from their usage routine?
Another thread (I don't remember which) had a link to a British government site, listing Harman's expenses and gross revenue: Ilford makes around $20 million a year in sales, for profits of $1 million. My local Costco location (I worked there at one point) does more sales than that in a year, and it is the smallest Costco in Canada! Some of the large Costco locations do more than that in less than a month. Even if they wanted to, Harman/Ilford probably does not have the funds to purchase the patent, never mind start-up costs for production.
As for hoarding, I think this is understandably bad long-term thinking: photographers are justifiably scared a favorite product will disappear and purchase a large amount to ensure they can continue to use it. The company sees a short-term blip of increased sales followed by a long-term drop in sales as photographers use their stash rather than purchase more product. However, people dropping film as their medium of choice and photographers using their stash look identical to a balance sheet. It would be much better to rotate stock, so purchase 100 rolls and when you use up 10 rolls, buy 10 more rolls and put them at the bottom of the pile. This way, product demand continues and you keep your stash.