I have to agree with the first part of your last paragraph. Leah eventually comes to look up to Esme as a surrogate mother & close friend. Esme also thinks of Leah as a daughter even though they aren't actually related. That's probably one reason why Leah sacrificed herself. Another reason could be Seth's demise. Leah saw him die & probably felt like she had nothing left to live for. Leah doesn't need to stick around. Sue can take care of herself & anyway, Sue has Charlie to look after now. But watching Leah sacrifice herself was rather sad. Anyway, we must recognise this: Leah died a hero's death & her sacrifice will not be forgotten.