After the failure of both (or all) packs, with the aircraft sealed as well as it can be, what source of ventilation is there? Five or six hundred lungs pumping away in a sealed tube doesn't sound like a good idea.
True the air will become quite stale. However in order to combat this problem, aircraft are equipped wtih recirculation fans which pushes ambient air through the distribution ducts to the eyeball outlets above the passenger seats. On the older Boeings, this was called the gasper fan.
This following is an exerpt from the Boeing 737 NG Manual.
The recirculation fan system reduces the air conditioning system pack load and the engine bleed air demand. Air from the passenger cabin and electrical equipment bay is drawn to the forward cargo bay where it is filtered and recirculated to the mix manifold. The fans are driven by AC motors. Each recirculation fan operates only if the respective RECIRC FAN Switch is selected to AUTO. In flight, the left recirculation fan operates if both packs are operating unless either PACK switch is in HIGH. The right recirculation fan operates in flight if both packs are operating unless both PACK switches are in HIGH. On the ground, the left recirculation fan operates unless both PACK switches are in HIGH and the right recirculation fan operates even if both PACK switches are in HIGH.