I predict that WCMs will take it over fully in future, and will buy the fleet of E400MMCs from Stagecoach to run it.
It'd make logical sense for it to just become a full WCM route. At this point it's just a WCM service branded as Stagecoach anyway.
I don't really understand what value Stagecoach see in keeping the X19 though. It surely can't be making them a lot of money if they are now paying a lot to WCM to operate the route. In the 2 other instances of Stagecoach subbing out a route, the Cambridgeshire 904 & Worthing 7, there is a strong Stagecoach network around the route and so there is some benefit for Stagecoach to have the route subbed rather than dropping it. There is also some wider benefit for them to not have a competitor in these area and subbing out the route protects against that.
The X19 though, they only have the distance express routes, and the 4. The X19 barely links up to the express routes, and I can't see many people wanting to connect between the 4 and X19. There is no benefit to them either to keep an competitor out of 'their area' as it's not their area really.
It doesn't fit 'perfectly' into WCMs area but certainly fits more with their area than it does Stagecoach and at least there are some half decent local connections between the X19 and 15/17.