Appart from the fact that Warwick and Irelia are in the works for gameplay updates.
Visual updates HAVE EVERYTHING TO DO WITH UPDATED SPLASH ARTS.
Gameplay updates don't require a splash art when there are more important issues at hand.
Gameplay AND visual updates normaly require a new splash art, sometimes it's the level and scope of the update that dictates the necessity of an updated splashart.
Taric and Poppy aren't getting "reworks and relaunches", they are getting proper re-releases, as if a new champion were to be made.
Anivia got an updated splash art AND A TEXTURE UPDATE because her art is on par with her visual identity in game, her slight VU and the fact that her splash is nothing extraordinary(meaning it represents her ingame identity pretty well) means fewer resources were invested compared to something like Sion.
Compare it to Sivir, her kit remained pretty much identical, but her artistic values skyrocketed through the roof. Everything's amazing about her, and her splashart is out of this world.
Warwick and Irelia are getting the gankplank treatment, and if I'm not mistaken, Irelia gets a relaunch, but not Sion and Poppy's level. Corki got VFX updates and a slight texture update, pretty sure his splash is coming up soon. Kennen, Rammus and Swain have been left stagnant for quite a while, perhaps this means there's stuff coming down the pipeline, same with Gankplank, he was left to rot and now he got VU,GU and a new splash.
PS. " I would expect them to get new Splash Arts when the rework comes" - You don't expect something like that. It's a given, it's for granted. Why would you think they'd get full blown re-releases and not have updated splash arts? They're getting the same work as a new champion would(perhaps even more) who on their right mind would leave them with the splashes from season 0.