I don't know if I would jump to the conclusion that it's all slash fans and Sheppard/Mckay shippers who disliked Keller and McKay/Keller because I never shipped the boys at all and I still disliked the second pairing and Keller.

Sometimes people just don't like what you like. Sometimes people do not like characters. Sometimes people do not like ships and it has nothing to do with what they do ship (I don't really ship Mckay with anyone in truth, so it was never about Keller getting in the way of anything.)