Well, they'd have to either be intending a horrendously tragic vision where Sam and Dean are going to gradually, inevitably decay into the very things they hunt or have some convincing and not hand-wavey explanation for why the established canon on how ghosts work and established characterization (for Dean especially) on the natural order and how what's dead should stay dead don't apply.
I think Sam and Dean becoming reapers or some such works better as a version of that scenario. It puts them in an immortal in-between state with a function without as much world-building awkwardness to work around.
Re: What would be a happily ever after for SPN?
Well, they'd have to either be intending a horrendously tragic vision where Sam and Dean are going to gradually, inevitably decay into the very things they hunt or have some convincing and not hand-wavey explanation for why the established canon on how ghosts work and established characterization (for Dean especially) on the natural order and how what's dead should stay dead don't apply.
I think Sam and Dean becoming reapers or some such works better as a version of that scenario. It puts them in an immortal in-between state with a function without as much world-building awkwardness to work around.