My guess is that if Perez is going at the end of the season the second seat goes to Yuki. Unless they buy out someone else (which I seriously doubt) there really aren’t any other good options for RBR. Lawson isn’t better than Yuki, Colapinto is off the table, Bottas has ruled out RBR and the rest are unlikely to be better than Perez. It’s either Yuki or Perez.