Good PR always sells unfortunately Stoffel has been mentally shot to pieces and needs time away from Mclaren. Mclaren's situation has not helped him either but he is better driver than Lance Stroll to be always bringing up the rear of the grid
He's thinking that his talent has been tainted by an impossible situation at McLaren and that he has no money or backing from one of the big teams so he has to go compete in another series because
a) F1 fans, press and teams are fickle. If he sits on the sideline for a year not being talked about by June of 2019 no one will remember him. If he's driving up a storm in another series then he'll get talked about and might get a foot in the door.
b)it's quite possibly it for him and F1 so he needs to establish a career elsewhere. Getting involved with Mercedes in a potential up and coming series is a no brainer. It would not in the least bit surprise me if Esteban Ocon replaced Gary Parfeet at HWA mid season in Formula E either. As soon as it becomes absolutely certain he's not going to get an F1 drive then he'll need to go somewhere.