Honestly i felt exactly the opposite. I wondered if LJG could be as good as SJK. And I love SSK and think she's…
Lee Joongi is definitely one of the best actors we can get to replace SJK. He's been in many historical dramas and his acting is truly phenomenal. I'm glad that he was the one chosen to replace SJK, for I can think of few who fit better than him except perhaps Gong Yoo, off the top of my head, (but he doesn't really fit the younger-ish Eunseom and Saya vibe). Even his fellow cast members praised him -- Baekhyun spoke about how he was nervous during his death scene in Scarlet Heart Ryeo because he wasn't sure whether he would be able to cry, but acting with Lee Joongi helped him be able to enact that scene flawlessly.
I'm happy we're getting a season 2, and I have full confidence in Lee Joongi's assimilation of Eunseom/Saya roles, but... As a few others have raised, Shin Sekyung being cast as Tanya is what I'm a bit more doubtful of. I admit that I've only watched two of her dramas (Girl Who Sees Scents and Bride of Habaek), and I've heard that she does better in historical dramas ? but from those two dramas I've watched I found her acting really dull, her expressions monotonous and her delivery bland. I'm sorry, perhaps this will be her breakthrough role, perhaps I shouldn't judge so harshly before the drama has even aired, but I'm conflicted because one of my favorite actors is being paired with one of the actresses I'm least enthusiastic towards. (Let me clarify that I do not hate Shin Sekyung, I don't even know her as a person and she doesn't know me either so how can I even hate a random stranger? Rather I find her acting unappealing. Who knows, maybe after watching season 2 I'll have a change of heart and starting singing her praises???)
Same, I love Lee Joongi ever since I saw him in Scarlet Heart Ryeo and I have full confidence in his ability to portray Eunseom and Saya wonderfully, but Shin Sekyung... I dropped both her shows (Bride of Habaek and The Girl Who Sees Scents) because I found her acting really dull and for some reason it just never clicked with me. And since Tanya is a nuanced who has multiple layers to her personality, based on what I've seen of Shin Sekyung's past performances, I already have a strong feeling that I won't be liking it... but we'll see, I'll try to be less biased (?) when watching the show. Perhaps Shin Sekyung is better at historica dramas than contemporary ones? Although very unlikely as they both have already have dramas set in 2023, I was hoping for Han Sohee or Shin Hyesun to undertake the role. Secretly, I thought that Kim Sohyun would be able to pull it off as well if not for her vastly too young age as she is, after all, the "Sageuk Goddess",, and has many historical works under her belt. And this was just my own very biased preference but I also secretly hoped for Kim Hyeyoon because... I simply like her acting, aha. I know, random reason, and again, she's also too young for the role.
(Let me clarify that I do not hate Shin Sekyung, I don't even know her as a person and she doesn't know me either so how can I even hate a random stranger? Rather I find her acting unappealing. Who knows, maybe after watching season 2 I'll have a change of heart and starting singing her praises???)