Drug companies will never stop R&D, though they may threaten to do so.
It's their bread and butter.
Once the patent runs out on a drug, all the companies have left is their new stuff. The patent-free drugs will be produced as generics. No extra money in that.
No, what will happen in the speculated case is your latter suggestion: Canada and other nations will be restricted or cut off altogether. Distribution of a limited supply will then become their problem. They will choose their own citizens first.
But in the meantime, shipping drugs to the US has become a very profitable cottage industry in Canada.
Regards--
Mary Anne