If you assume raiser isn't a donk then shouldn't you also assume that he has KK+ or AK as you earlier said he "should" have to raise 75 in that spot?

You can't have it both ways - that is to say you can't say hey it's a 55 so the raiser could have any kind of donkish hand and then from the other side of your mouth say that he will "probably be thinking the same as you". Would be nice though if your could have your cake and eat it too.