The incentive is saving lives. That's what the researchers are in it for. The research is in large part publicly funded. Pharmaceutical companies are just charging rent on top of the discoveries.
The best argument you can make is that private pharma competes to improve manufacturing processes, but you world also have that in a 100% public system from multiple countries competing/collaborating with each other.
Like I said, that research is already publicly funded, and even if it wasn't it's always worth it to make sure people live longer as a country because more people means more resources to accomplish stuff with.
The best argument you can make is that private pharma competes to improve manufacturing processes, but you world also have that in a 100% public system from multiple countries competing/collaborating with each other.