All it takes is for one business to come up with a cure to disrupt the market. If there's a cure, why opt for the ongoing treatment? Such a company will sell the cure for a very high price. Insurers still go for that cure. Maybe not in all cases, but at least the more problematic patients will get it. That company then has a big ace up their sleeve with that cure, and of course patents the hell out of it. Other companies try to come up with their cures so they get picked by insurers. In the meantime, the first company has its monopoly. Once that has run its course, and competitors are coming up with their cures, the first company drops their price massively to undercut the competition.

Diabetes treatment is very expensive for health insurance providers. A cure would be cheaper for them even if it costs 500k once.

And yes, cures are being made for diseases. See this for example. Another example is the ongoing research into growing islets out of stem cells and transplanting said islets. See the Vertex VX-880 trial for example.