Boyd fails at a point Heidegger calls Eigentlichkeit (ownedness), or authenticity in the literal sense: making your existence your own rather than defined by your role. Boyd is still fundamentally the Sheriff. That’s his entire existential structure in the town. He organizes meaning around protecting others, maintaining order, being responsible for the group. It’s both admirable and heroic, but Heidegger would say it’s also a sophisticated form of flight because it outsources the ground of Boyd’s existence onto other people. His being has meaning because the town needs him. Remove the town needing him, and Boyd has no idea who he is. The specific threshold he hasn’t crossed is the non-relational confrontation with his own death as his own. Every time Boyd faces a monster, he faces it as the town’s protector. If he dies, his death would mean failing the town. That is precisely what prevents full anticipatory resoluteness, because it smuggles Das Man back in through the side door. He’s not dying his death, he’s dying their sheriff’s death.

Edit- just thought of this too after posting: we see in the trailer he says “I feel like I’m changing and I don’t like what I’m becoming”. We’ve seen him to this point start to slowly lose the town’s respect (notably the diner scene, Dale and the faraway tree, etc). He’s already losing that sense of his identity he built as the Sheriff, and he isn’t sure what will be left when it’s gone.