The difference between whether a character flaw or plot point should be turned into an arc vs. just cut entirely lies in how that meshes with the rest of the character it relates to, as well as the story it's in as a whole.
For example, Adrien continuing to chase Ladybug despite being rejected over and over is something that definitely should be an arc. His core character flaw is his excessive optimism. He keeps hoping that things will change, and that results in not knowing how to let a relationship end, or when to stop pursuing a goal. This, combined with his power of destruction, lends itself perfectly well to an arc about how sometimes it's healthy to let things die, despite the pain it causes in the moment. It's for the same reason that I roll my eyes whenever people say Adrien facing Gabriel would've been too painful for him. The fact that it's painful and bittersweet is the whole point.
On the flipside, the sentinonsense and the Big Lie are two things that should just be cut. Between Adrien being restricted by his father and his power of destruction, the set-up is about Adrien escaping from the restrictions in his life. Being a sentimonster takes those restrictions and makes them inescapable. Adrien will forever be vulnerable to having his free will hijacked, and that just doesn't jive, narratively. Sure, you could have a moment where Adrien severs the bond between himself and his amok, but at that point it's like he was never a sentimonster to begin with, so why bother? It doesn't add anything to Adrien's character; it's a shocking twist for the sake of having a shocking twist.
Same with the Big Lie. For Adrien, the most narratively coherent moment to confront everything his father has done to him would have been the S5 finale. Moments like him destroying Gabriel's statue or yelling at an akuma with his father's face ring hollow, because, like. Gabriel's dead. There are no stakes anymore. Adrien doesn't risk anything and doesn't have anything to lose. Learning that sometimes it's healthy to let something end doesn't work if that something already ended. It's the storytelling equivalent of "You can't fire me, I quit!"
As for Marinette, remember how I said that the set-up for Adrien's story is about destroying the restrictions in his life a couple of paragraphs ago? Well, now Marinette is the one restricting Adrien by lying to him and telling him what he's supposed to think. Symbolically, Marinette's role in Adrien's story is about creating (natch) a new, healthier future after the old, unhealthy status quo has been destroyed. This puts her fundamentally at odds with Gabriel. They pretty much have to be polar opposites for this to work. Instead, nor only are they very similar, but Marinette has now taken over Gabriel's old role as Adrien's villain. Why the fuck would you do that to your main character and romantic lead?!
