The filament of the light bulb will get very hot. This will encourage a chemical reaction with most gases that are surrounding that filament - and the result is that the filament burns out. If the filament is in air, it combines with the carbon of carbon dioxide in the air, and the filament disintegrates. But argon is an inert gas - almost nothing reacts with it. So the filament takes a very long time (theoretically infinity) to burn out. But the bulb cannot contain 100% argon: 99.9% is typical; the remaining 0.1% being air. The bulb manufacturers can control the 'life' of a bulb, based on that principle: they do not want their bulbs to last forever!