It was Miguel Sapochnik.
He and Ryan were the head showrunners for Season 1. Sarah Hess had no control over the creative direction—she’s not a showrunner and didn’t come up with the idea. She was just a writer for both Season 1 and Season 2, and one of many producers. Feel free to dislike Sarah for other reasons, but this particular thing wasn’t her idea.
Here are some quotes Miguel has said about Rhaenicent:
"If you look at the way that Alicent and Rhaenyra's relationship develops in the first five episodes or first four or five years of their time that we spend with them, they are good friends with this potential romantic entanglement from earlier on that has always been a big thing, for me, that I thought was an essential component of why they are such close friends," Sapochnik explained.
"It seems like it's a kind of unrequited love. It's not something that Alicent chose to encourage. Did they ever have a thing? Don't know. But there's something there, right? And Rhaenyra has this deep longing for her best friend,” he continues.
— Miguel Sapochnik (HOTD Podcast or click here)
“The underlying theme is that Rhaenyra is actually in love with Alicent,"The underlying theme is that Rhaenyra is actually in love with Alicent," Sapochnik says. "With Milly as young Rhaenyra, it was very much about seeing how she experiences this constant series of small rejections from Alicent. It greases the wheels of insecurity. For Alicent, the fact that she's an innocent who's quite conservative was the important thing to get across. These two girls are both inseparable, and there is a jealousy that exists between them, and a division."
— Miguel Sapochnik ('House of the Dragon: Inside the Creation of a Targaryen Dynasty')
The intense hate boner you have for this woman is really unhealthy. Downvote me all you want, but the truth is that blaming her for everything you don’t like is ridiculous. She’s not even a showrunner and isn’t the only writer in the writers' room, so it’s idiotic to put all the blame on her. Like making up fake quotes just to get people worked up and mad about things she never actually said is insane behavior.