"Look, I understand that she'll never be my girlfriend and I'm fine with that.
However, I can't see what's wrong in wanting to keep hanging out like we used to."
There is plenty wrong with that approach.
She told you she does not want a relationship. You need to add "at the moment" to that in your mind. BPDs can get very attached and they can also change their minds/emotions very, very quickly. Just because she does not want to be your girlfriend now does not mean she won't be convinced that you are getting married and having kids a little down the road. While that might not sound like a problem now, you may not to marry her and have kids after you have had a full taste of her BPD behaviors. In that case, you will most definitely be the bad guy who led her on and then abandoned her. She may lso deny ever having told you that it wasn't going to be serious.
Generally, it is a good idea to add "at the moment" or "right now" to ANYTHING a BPD says about his or her feelings.