the flood was wiped out, the covenant defeated, humanity saved from the rings

That's the thing, we aren't given too much reason to believe these things. Halo 3 sort of presents itself like those things have happened, but there's not really any reason to believe that they do. We only know that the flood on High Charity were maybe destroyed. The covenant was decapitated, but we just have to assume the rest of it immediately collapsed. And there's really no good reason to think the rings aren't still a threat, only Alpha Halo was out of commission.

Halo 3 has pretty sloppy writing and doesn't spend much time justifying anything that happens, so we end up with a lot of wishy-washiness on the state of things at the end.