Imagine you own a company that manufactures the single best coolant ever known.
This coolant changes he world for the better, including refrigerating medicines to save lives in Africa, etc. etc.
This coolant makes your company $$$BILLIONS of dollars
Now... imagine the patent is about to expire, and once it does EVERYONE can make it ...
What do you do? do you sit back and watch a few billion dollars income go away?
Or do you invest a few million to start an environmental movement to ban your own product?
And the only thing that is allowed to replace your product is an inferior coolant that is highly corrosive and is significantly more harmful to “the environment”
Or, alternatively, somebody figures out your coolant might actually not be all that neat an idea ~ so at the moment it looks like maybe the Iceland supervolcano may be blowing off some fluorine we missed?
By the same reasoning no one would continue to manufacture a drug after the patent expired.
They’d make up some horrible side effect and manufacture inferior drugs to replace it.