We still don't really know anything about that archway.
Just vague hints.
Lupin THINKS he knows something and honestly believes that Sirius is dead.
Still, it's in the department of mysteries. It's a mystery.
Probably no one knows.

The mood, the writing style, the absence of clues, make me believe
that Sirius will stay dead. But who knows?

A gingerbread ending? Any death in which they never found the body is fair game.
What might JKR do? She's sent out so many contradictory clues,
I wouldn't put anything past her.
Like Narcissa said, "There's nothing I wouldn't do any more."