The GaaS push wasn't even his call. That seed had already been planted before his appointment as SIE's CEO back in 2019.
For example, Helldivers 2 and Concord were greenlit in 2015 and 2018, respectively. Another one that started in 2018 was Horizon Online, back when Guerrilla hired the Rainbow Six Siege lead to helm the project. We know that Factions - in its various incarnations - has been in development for just as long as all of the previously mentioned titles. Hell, even the deals to get Destruction AllStars and Predator: Hunting Grounds made were already in place before the start of the Jim Ryan era.
So the live-service initiative was already a thing by the time Jim and Hermen took on their respective roles in 2019. And while Hermen not only carried on with the development of the aformentioned titles and started new ones - Fairgames, Twisted Metal, Firewall Ultra, Insomniac's MP game, London's MP game or Deviation's game (which was cancelled but still) - he didn't do it at the expense of singleplayer games, as several of these had been greenlit over the past four years of him being the head of first party: Death Stranding 2, Marvel's Wolverine, Ghost of Tsushima sequel, a new Astro game, The Last Of Us Part III, Bluepoint's original game, FromSoftware's new IP, Ballistic Moon's horror game (Bates), Firesprite's horror game (Heartbreak), the leaked sci-fi Sony WRPG from last year...
None of these MP games have come at the expense of any singleplayer game.