The issue is with what drug R&D looks like. You invent some new compound you think will treat X because it has a similar structure to other compounds that treat X. Now you need a decade or so of trials to prove that it actually treats X, that it doesn't have side effects too severe to stop people from taking it for X, that it doesn't also silently cause some kind of obscure cancer, and then it might get approved (and if it doesn't that manpower and money was wasted) and the exclusivity time granted by your patent is how you turn a net profit from the last ten years of work because it's much easier for another company to spin up a factory making X than it is to get X approved in the first place so anyone else making the drug can charge less to cover their much lower costs in getting it to market and will eat the lunch you spent the last decade+ cooking.

Unless you intend for medical R&D to be done purely under public funding, which is an entirely different scenario than just "no patent law."