Monday
Monday's origins trace to ancient Mesopotamian civilisation, where the seven-day week first emerged approximately 4,000 years ago. The Romans formalised Monday's position, naming it dies Lunae in honour of the moon. This represents an operational history exceeding that of most human institutions, religions, and certainly most individual beings.
Projections suggest Monday will continue to function as long as human civilisation maintains the seven-day week, and potentially beyond. Even proposals for alternative calendrical systems have failed to gain traction, ensuring Monday's continued relevance for the foreseeable future of organised society.
Thanos
Thanos, as an Eternal of Titan, possesses considerable individual longevity. His lifespan exceeds that of typical mortal beings by several orders of magnitude. However, his fictional existence depends entirely upon the continued commercial viability of the Marvel franchise and audience interest in his character.
Cultural analysis suggests that whilst Thanos currently enjoys peak relevance, all fictional characters eventually fade from collective memory. Characters from popular entertainments of previous centuries now languish in obscurity. Monday faces no such obsolescence risk, requiring no continued audience engagement to maintain existence.