The sunset is a kind of finale, though not one as final as death. It's the finale of a day, which may indeed be remorseful as A.E. Housman wrote but need not be. Remorse is understandable at the end of a day, or a life, to one who is thinking at the end of one or the other, because it's not uncommon to feel remorse for what was done or was not done which could have or should have been done. Humans being what they are chances are excellent that they should have or could have done something and that something they did was wrong.
What seems to be isn't necessarily what is. To be is to exist. What exists may seem not to exist. You get the picture (or at least you get what the picture seems to be).
"Let be be finale of seem." Let the end of our efforts to determine what seems to be, be? Let what seems to be, to us, be? Or, let it be, in the sense of give it up, forget about it? Give it up, this tendency we have to distinguish appearance from reality, to insist on a thing-in-itself that we cannot know? Give up the belief that what seems to be reality isn't really reality (the really real)?
I'm impatient with metaphysical and ontological concerns, i.e. the issue of Being and, that other favorite, Nothingness. That may be a failing on my part. I'm unconcerned with questions regarding what it is, in all cases, to be or not to be (was Shakespeare having a bit of fun with philosophers when he wrote this speech of Hamlet? I find myself hoping so). The fact that we make mistakes sometimes leaves me unimpressed. It hardly seems grounds on which to question all we interact with naturally, by living, with considerable success, much less envision some kind of place apart from the world on which all truth and beauty depends.
So, I would interpret Stevens as saying that what is, is in the end what seems to us to be. In the end of the day, in the end of us, you and me. Let it be so.
"Things Merely Are" is the title of a book by Simon Critchley about philosophy in the poems of Wallace Stevens. It seems a less than hopeful phrase, but it's an assertion that renders a good deal of speculative philosophy superfluous. And, if that was what Stevens was attempting to say, in his poetry, it's something few other poets have said, I think. Yet there's unquestionably beauty in his poetry, just as there is unquestionably beauty in things of all kinds.
Consider sunset, especially as pictured above. The sunset merely is, of course, but though we don't cause the Earth to revolve or the sun to exist it is what it seems to be to us, the end of a day, and though it can seem to be, to us, a splendid end or a dismal, dull one, it's nonetheless an end of something.
We're edging into Autumn, now, and in that part of our Great Republic in which I live the leaves will turn glorious and then wither and fall. Crops will be harvested, vegetation of all kinds will stop growing, weather will turn cold. As we've known since ancient times, we wither and die like other things in the world do. But some of the most striking sunsets I've ever see have been winter sunsets, when the world seems, and is, dead. Those sunsets merely are as our deaths merely are.