Very probably. AIUI Ryanair's routes from Prestwick are largely a means if rotating frames through their maintenance base there. ie the frame which operates the return flight to Marseille is likely to be a different one. So the flight revenue is less important, just avoids an empty leg.
I wonder if the spare rotations will be used by Easyjet to increase flights to more profitable destinations. They've been saying that bookings on their holiday arm are above expectations