According to Assistant Community Manager Gina Bruno on the official forum, Lead Content Designer Rich Lambert has stepped into Sage’s vacant position:
We can confirm that Paul left ZOS after the launch of ESOTU on console. He, along with rest of the team, are very happy with how the game is doing and we wish him the best in his future endeavors. ESOTU continues to be in great hands with Gamer Director, Matt Firor and new Creative Director, Rich Lambert, overseeing development. Previously, Rich oversaw all content in the game as Lead Content Designer and worked on The Elder Scrolls IV: Oblivion. We’re very excited to have him take on the role as Creative Director!
Sage isn’t the first high-profile dev to depart ESO in recent months. Lead Gameplay Designer Nick Konkle left ZOS for Riot Games earlier this spring; he was replaced by Chris Strasz. Combat Gameplay Designer Maria Aliprando apparently departed in May.