Well, the OG Thomas and William are meant to be like Wanda and Vision, mainly back up by some details in Englehart's Vision and the Scarlet Witch and AU comics like The Last Avengers story.(Actually I am very interested in Billy meeting the AU Sorcerer Thomas.)
It probably won't get addressed like why the magic is shifted from Thomas/Tommy to William/Billy.
I think it's probably Jonas already taking the Vision role in the team, it would be kinda redundant if the twins are a magic user and a phaser/density-shifter.