The NANOBOT FACTORY breeds self-replicating assemblers that strip and refit obsolete units in the field, free of charge.
The replication bound is mathematically proven to hold in all anticipated conditions.
Requires SYNTHETIC ECOLOGY.
If the bound fails, a grey tide doubles every 5 turns. It cannot cross the sea. Units left on it are lost; a city under it for 10 turns falls. The factory never refits again.
SETTLERS are immune: order them to CLEAN POLLUTION on a goo tile (2 turns). A NUCLEAR strike clears all goo connected to the blast.