Agree here, even as a pearlshipper, but I think, even for a "sibling relationship", Ash and Dawn have friendship on a different level compared with Ash's relationships with Misty and May. Misty had her "I'm better and more mature than you" thing, meanwhile May is really like a sister to Ash. Dawn is different because she is more like a partner than the other two, she have a complicity with Ash that I never saw on Misty and May.I do agree that Dawn and Ash have a slightly different friendship than the others....but I feel like they have more of a sibling relationship.....and everyone knows that kind is not always the best.
Not as a girlfriend, but I think Dawn could easily turns into Ash's best friend, aside pikachu or Brock. Or she already is.