Gonna link another post of mine:
I don't think it's unreasonable to assume that Voldemort was going through the same cognitive biases that kept him from annihilating the Order of the Phoenix back in the first war once he went full-time Voldy. He left Harry ways to escape for half-formed irrational reasons. On some level he really wanted to have an opponent to play against, in spite of his perception of the prophecy and the risk it posed to his immortality, and that hindered his judgment.
I'm not saying Voldy didn't have a backup plan that has already happened right under our noses. He may have. ... I'm just saying that if he didn't have some sort of backup plan and has indeed been straight-up defeated, that won't be a failure of the narrative or of Voldemort's characterization. His underlying sort-of-subconscious need to have an opponent, to have someone to play against, already previously led to the biggest mistakes of his life. Such a thing could easily happen again.
It was already established shortly before this that Voldemort is not a perfect rationalist. He has an extremely irrational "need" to have an opponent. Having gradually come to awareness of this failing in his thought processes over the decade he was trapped in his Horcruxes doesn't mean that he's readily noticing it when it happens again or even wanting to act against it (since the end result of acting against this bias would be an eternity of boredom--his ultimate victory means he's back to only finding joy in killing idiots).
There may be a secret plot here. It's possible. But EY took time to show us that one of Voldemort's only weaknesses is this desperate need to have someone to "play against," and if there was no secret backup plot, it won't be unsatisfying to me. On some level, whether he realized it or not, Voldemort wanted Harry to escape. No part of him assigned any probability to the possibility that Harry could and would win utterly and absolutely right there.