If absolutely no other variations of this part of the spike exist then the antibody could remain effective and our use of it wouldn't materially push the virus to mutate.

But, if any (survivable) variations do exist, their survival advantage would grow tremendously once this antibody saw widespread use in treatments, which would lead to the variations likely becoming dominant quite rapidly thereafter.

Still, there's no reason not to keep searching and studying. We may be able to make an effective treatment based on this.