Costs of drug development are primairly driven by the need to adhere government regulations, not because of the R&D costs. With simple adjustments, whole new drugs can be created. Those costs are actually quite low compared to the administrative burden of bringing a drug to market.
In light of this, government has to grant some form of protection. Otherwise, as you so rightly conclude, no new drugs would come to market. But to mark these innovations as a result of patents, is the world upside down.
Lift the regulatory burden and you have no need for patents in the drug industry. Patents are anti business, dispite the fact that they are being marketed as pro business by businesses that own them.