They're planning to discontinue Falcon 9 to free up factory space for BFR production while continue launching with their existing stock of reusable rockets. So there's quite a bit of risk there if BFR ends up getting too delayed.
They will not stop producing them until they are sure they can pull of BFR. The CTO has already said that Falcon 9 will be available as long as people ask for them.
By that point where they can no longer produce F9 they plan to have a fleet of Falcon 9 on the books that could continue for quite a while.