Take, for instance, Harvoni -- a drug introduced in late 2014 which cures Hepatitis C following a single course of treatment. It has done something like $100B in revenue for Gilead Biosciences, and, minimally, earned them $7-10B in profit. (Possibly much more than that.) Its pricing was scandalous, but that's not the issue here; the point is that it was unequivocally one of the most profitable drug launches in history.
Sure, the eradication of Hep C might make it "unsustainable" -- but it's not as though there's a lack of other diseases or maladies to contend with. Take the profits and plant new seeds, buy new technologies, develop new drugs. Besides, the research and development of new drugs has never been a stable business model, and never truly sustainable off one discovery, on account of patent expiry terms, generic competition, etc.