Worthy to note that Benedict did not fall in love with Sophie because she was the LiS, he fell in love with the LiS because she is Sophie.
He takes an interest in the Lady in Silver, feeling devastated and lost in the absence of her but when he later recounts the experience, he confesses he has never known her at all. He doesn't remember what she looks like, he has vague sketches of her- each leading him astray in his search and abruptly decides his search is pointless - his search, consisting of publishing his woes in Whistledown, attending soirees and basically participating in society and conforming to what is expected of him. When he participates in the society that is very quick to shun people for being any different, his search yields him nothing. He is quick to give up, comforted by an old fantasy and eager to move on.. and that is when he meets Sophie.
Benedict meets Sophie in the most unlikeliest of ways. He's drunk, vulnerable and has no interest in well, living his life as a gentleman anymore. He sees Hazel in trouble, he sees Sophie rescue her - putting herself in danger in place of her friend and he's overcome by this swell of righteousness, confusion and anger and decks Cavender in the face before he does more harm. He doesn't know Sophie. He doesn't know Hazel. He knows his gentlemanly friends, but it is when he is confronted with this uncomforting reality where his gentlemanly friends and the rest of the Ton see maids as disposable, forgettable and exploitable, he really truly sees it all. When he is detached from the fantasy, ever-spring land of Mayfair, he sees the cold cruelty and wishes to colour it with kindness.
Sophie is.. disturbingly honest. She doesn't thank him, not yet, because he's cost her her employment. Sophie knows how good he can be, but that was before. That was when she had pretended to be some noble woman and he gave her dance lessons. She is only a maid now. Benedict pleads with her, desperately and convinces her to come with him. He promises he'll find employment for Hazel as well, knowing how important it is to Sophie. He goes the extra mile, all for a maid, because he cannot possibly grapple with a world that is so deeply broken and unkind. He's not used to it, for he's used to bright colours, pleasures and familial comfort. Sophie is wary because she knows how people tend to treat maids - even the kindest of gentlemen cannot be trusted, constantly waits for Benedict to simply discard her somewhere, but.. he doesn't. He fixes his kite for her. He encourages her to leisure. He doesn't tell her he's .. insanely injured, therefore subjecting her to the role of a housemaid yet again. He writes to all his friends on behalf of Sophie and Hazel.
And he's also drawn to her. Sophie doesn't know. Benedict loves her whimsy, her enthusiasm, her curiosity. He notices all the small things, even noticing her change of clothing. She's witty, she humbles him a lot, she is achingly honest and doesn't see vulnerability as a weakness (going as far as to talk about how she flew kites in her childhood and how important it was to her - she never had to tell him all of that!) and genuine.
Benedict confuses her with the Lady in Silver ("When we were at My Cottage, did you ask me about feeling out of place?") not because he thinks the LiS is Sophie, rather he recognises Sophie in the LiS, subconsciously. He now associates genuineness, openness and kindness with Sophie, to the point where he is visibly offended when Miss Hollis (indirectly) calls her a fool. His memories of LiS are so tied to Sophie, again, not because he thinks the mystery woman is her, but because he loves Sophie's kindness so much, he lets it superimpose his memories of LiS ("Lakes, deep waters", "French", basically the whole Miss Hollis scene lmao).
The LiS is a fantasy, because she can never be seen again. She was a temporary, passing ember of goodness in a society that rewards gossip over truths and perpetrators over survivors. Sophie is a reality, a permanent ember of everything good in the world. Benedict's reality invades his fantasy so much until he can only see the reality of it all - that he ought to be with Sophie, whatever it takes. Benedict's choosing of Sophie over the LiS is not a reality vs fantasy thing, rather him considering reality as the fantasy itself, taking reality as the one he is meant to strive for - instead of trying to construct a dreamscape to escape into - this thing that can "change him entirely".
