How much damage could a total landing gear malfunction do to an airliner? Could the runway wear a hole through the plane?
According to Belly landing:
During a belly landing, there is normally extensive damage to the airplane. Belly landings carry the risk that the aircraft may flip over, disintegrate, or catch fire if it lands too fast or too hard. Extreme precision is needed to ensure that the plane lands as straight and level as possible while maintaining enough airspeed to maintain control. Strong crosswinds, low visibility, damage to the airplane, or unresponsive instruments or controls greatly increase the danger of performing a belly landing. Still, belly landings are one of the most common types of aircraft accidents, and are normally not fatal if executed carefully.
It is possible that the friction from a belly landing could wear a hole through the underside of the fuselage. However, even if this were to happen, airliners carry the passengers and crew in the upper half of the fuselage, so nobody would be likely to notice (from inside the aircraft before emergency evacuation).
This is highly unlikely -- there is a sturdy keel beam at the bottom of an aircraft's fuselage, similar to a ship's keel. This beam serves as the main fore-and-aft member in the aircraft structure (its primary function is to transfer bending loads to the center wing box), and will take the brunt of the damage from a belly landing once the belly skin wears through.
Of course, total belly landings are quite rare compared to partial gear landings, which are much kinder on the airframe itself (usually scraping a wingtip and/or an engine pod bottom cowl).