I think Southwest will be more concerned with their own routes than with competitors.
They are successful with their NQY-BRS-LBA (with PLH pax also able to access LBA with a change at BRS) against Flybe's also successful EXT-LBA, so I can't see them having sleepless nights about their PLH-BRS-MAN route, now that Flybe is appearing on that scene.
Remember a significant amount of the traffic is generated between BRS and MAN anyway, and some rotations start/finish at JER as well.
More choice for the southwest region as a whole is the way I see it.