The difference between (a) an existing, off-patent molecule and a (b) patentable variant of that molecule is somewhat trivial compared to all of the other costs

The difference between (a) an existing, off-patent molecule and a (b) patentable variant of that molecule is somewhat trivial compared to all of the other costs of bringing a drug to market. But to address your specific question, generics are drugs that come onto the market all of the time without patent protection.