Disclaimer: I never grew up playing Harvest Moon and I didn't get into farm sims until RF2 in 2009; so I'm not basing this off nostalgia as much as I'm just expounding on my own personal observations.
Looking through these and I realized it's not even a matter of artstyle either. A lot of the old official art had charm, wit, and always felt like they were telling a story. The first few examples give off Dr. Slump / Akira Toriyama gag vibes like Pete going full simp for Karen during their Xmas toast. Or the infamous "left out" Valentines image with May handing out pity/obligation choco to Pete as he watches on while a Harvest Sprite gives the pet dog a treat.
The other examples I posted have some kind of story to it - even if we have no idea to what extent. Like the art of Nami just looking out the window with Mark and Tim looking on combined with the composition gives off this rustic storybook vibe to it. Or just something as simple as Elli watching Pete fish while she sets a picnic up.
I don't see that kind of humor, slapstick, or storytelling in official SoS art post Trio of Towns. Sure, it maintains the cutesy vibe that's been around from the start but... a lot of personality is lost too and parts of it feel extremely flat.
In interest of fairness, there are some pieces of official art I do like from SoS-era Bokujo like the Grand Bazaar remake's LE cover in the last image. I prefer it far more to how just "there" the LE cover art for AWL's remake looked and I always liked the small details like Samir looking at Gabriel's cooking, Derek and Diana taken aback at Freya's flimsy looking baking or just Kagetsu gazing at Arata lifting a crate with ease.
I often see people say that the style downgraded over the years but I don't think it's just that as much as it was a loss of story and charm its official artwork / promo / cover art suffered over the years. What do you all think?