Jarvan IV is only weakening Demacia's position by devoting all his resources to hunting down the mages in Demacia, and I imagine Swain's gonna catch wind of it and capitalize on it. It'd be really interesting if the follow-up to the Lux comic were to involve Lux learning that a Noxian invasion is coming and that she and her refuge mages need to do something about it, as Demacia won't win if it's fighting itself.
Of course, that's just speculation. Swain could also wait until Demacia has crippled itself sufficiently through infighting before he strikes. Still, that doesn't mean he couldn't send in a few raids.