My partners bring a lot to our relationships. I find it a lot harder to understand what they see in me.
My sense is that he is talking about the modal relationship in our society, that is mono, and in which my understanding is that people often (I would say at least 10% of the time?) do in fact have the "trading up" nature. That being the case, I think it's better for the participants in a relationship to be aware of that, and at what threshold to expect it? Having a moderately awkward discussion early on seems better than the heartbreak later.
This is coming from a very ask / tell culture perspective. I'm autistic enough (diagnosed, not slang / common use) that guess culture / relationships as imperfect information games is a distinctly negative experience. I don't find any "magic" in not considering bad outcomes or pretending that potential futures don't exist (the "happily ever after" expectation) or in leaving things unsaid.