She's not gonna fall for someone else. She's a Shichibukai and she's met a lot of men, very impressive and powerful men that she has no interest in.
She's only attracted to Luffy because he's different and "special." He's not like the other guys she has run into. He's immune to her charms and he's the selfless hero she thought didn't exist. He has conqueror's haki like her, too. They're also supposed to be the perfect match according to trivial things.
The only other match for her would probably be Garp but he's Luffy's grandpa. Luffy is his younger, smaller, dashing rogue version.
This isn't a shojo. Not everyone is going to paired up with someone. In fact, we'll be lucky if anyone gets paired together by the end of this. Oda may just decide to end it with no pairings, that the group goes on travelling with no indication of what the future holds.
That said, my point still stands. Hancock's feelings for Luffy has nothing to do with Luffy's feelings for Hancock. Just because Hancock may not find herself in love with anyone else doesn't mean that Luffy is obligated to stay with her. People often continue on after their loved ones move on with others. It's a trope called 'I want my beloved to be happy' and may come from knowing that their loved one loves someone else more, that confessing will ruin their friendship, or just that they know their loved one doese't return the feelings.
As far as writing goes, here are my biggest reasons for not pairing Hancock:
1. "A character can't confess to an oblivious character until they return the feelings on some level. That almost always means that the character will reject them." Luffy didn't even spare Hancock's feelings as he bluntly turned her down.
2. "If they are rejected, they will only have a chance if they a.) have moments together that reveals a different aspect of their character or b.) do something significant that changs their loved ones opinion of them." Hancock pretty showed more of herself and helped Luffy out significantly before the skip. There's not much left to sway Luffy's opinion of her. He's grateful for what she's done but that doesn't translate to love.
As for what you were saying, there are a lot of characters in this series that I could imagine would be immune to her charms. Could you honestly say that Zoro, Mihawk, Crocodile, etc would go all lovey dovey for her? Zoro didn't even go peek at Nami and Vivi at the baths. He might be more asexual than Luffy.
^Relationship goals, though.
^Zolu teas, captain first mate tea, zolu is real etc.