I'd rather see them hang it up and keep their pride and reputation instead of going on for another 10-20 years and becoming a joke like the Rolling Stones or the Who..or dare I say...Ozzy.
Ozzy is regaining his live voice. And I'm sure there's gonna be a last outing with Sabbath so there's no need for him to pack in.
The Who and The Rolling Stones are absolute rock n roll legends and still pretty good live these days. The Who are fucking solid... great tight live band. The Stones I shouldn't have to speak for.
Slayer... They're a bit of a childish metal band so I don't see them having as long a life as bands like The Who and The Rolling Stones.
Slayer haven't been worth much in years anyway - Christ Illusion had it's moments, and Diabolus had a bit of flavour despite being lame, but for the most part they've been total drivel since Seasons.