The whole premise of the notion is that managing the disease is more profitable than curing it. So it is not the ones who develop a cure that wipe everyone else out of the market but rather the other way around.

Consider a disease has an effective daily pill treatment where each pill costs $.50 to make. Someone comes along with a single pill that costs $1000 to make but cures you. The cure developers have just spent a lot of money on R&D and need to make a profit on the administration of the cure, so they need to price it significantly over their production costs. The established treatment on the other hand has a war chest and can recoup short term losses in the long run, so they can afford to drop their prices or even give it away for free for a limited time. Perhaps some segment of the population will pay a hefty premium just for not having to deal with the hassle of treatment, but most people don't have thousands to spend on such a luxury and will stick with the option they can afford right now, even though it will wind up costing more over the course of their lifetime.