This is unthinking and uses fallacy of edge cases. Brain transplants. Reality is generally a core of you remains. Yes some people completely change. But that is an edge case. This tries to be profound, but ain’t.
Even if it is an edge case, it's still proving that eternal love is unrealistic, no? You cannot properly love someone if they do not embody the things that you loved them for anymore. If you love someone for the core of them, that is still conditional; if they do not have those core traits that make them up, then how can you truly love them if they no longer possess that core? Like OP says you are merely loving a memory or an idea of who that person was, not the actual person presented in front of you anymore.