The system is closed basically as far as mass goes but its a open system for energy primarily from the Sun but in general fusion creates a open system vs energy and breeder style fission effectively creates a open system. Also mining of solar system creates a system that is effectively open for critical raw materials also of course nucleotide synthesis can be used if needed and if energy is abundant. A element synthesizer orbiting close to the sun can provide effectively unlimited amounts of elements for a price at a finite rate.

I know it sounds like science fiction but nothing I said is not doable starting with todays technology it a engineering problem.

The problem is EROEI cannot be solved simply and constrains the system. The real constraint is thus EROEI which is constrained by...

E=mc^2

This constrain is one we don't know how to beat yet.