I think this poem is more about the impermanence of Earthly things than it is about love being irreplaceable. The main theme is that while the concept of love seems infinite, love between people is not permanent. Neruda positions this theme against images like the sky and the stars which are infinite when compared to a love that has ended. The speaker is further conflicted because even if he wants to continue to love this woman, he can’t recreate the feeling.
Love is so short and oblivion so long.
The speaker says that his soul is lost without her because his soul is infinite and love is not. It is the “saddest poem” because the speaker can conceptualize what infinitude is but is unable to perpetuate this love. His actual writing about it is another attempt to make this particular love permanent.
On a practical level, this is just human nature. People grow apart and form new relationships. In this case, love has a kind of permanence but only through change. You might say that this means “love is irreplaceable,” but I’d say it’s more accurate to say “love is unrepeatable.” You can’t repeat love but you can replace it with something new. This calls to mind the phrase “if it ain’t new, it’s through.” To perpetuate love, you have to continue to be creative. There is creative promise and hope in that love is so ephemeral and original that it cannot be repeated. As soon as something is copied, it is no longer original or as genuine. But the poem seems to dwell mostly on the loss rather than love as constant renewal. Whenever you try to recreate feelings or situations from your past, you are bound to fail. There is sadness for this loss.