Think of it this way. It would still be rude to tell a person who is getting married “well, it probably won’t last” even if that person *did* wind up getting divorced. It’s disrespectful to dismiss who someone currently is by telling them they probably won’t always be that person. Fluidity exists for some people — in my case it wasn’t that my identity actually changed, I just changed what terms I used for it once I had access to better ones and more self-understanding. But asexual was a necessary step along that route for me, and it was a real experience while I inhabited that space. Anytime someone told me it might change, it was a way of them rejecting my agency in the matter, even if they didn’t mean it that way.