What I think is this:
You are mistaken to think that the "colonies" are worthless without Sol. Counting your 18 years, it's been close to half a century since the node was collapsed, it's the height of arrogance to think that the individual planets don't have at least a similar level of industrial infrastructure as Sol. If anything Sol would be closer to be a used up husk of a star system than say Delta Serp or something since it's been industrialized longer.
Who said anything about the colonies being worthless without Sol?
I'm honestly not sure we can continue this discussion until you begin reading the posts aimed at you. This is not a matter of industry alone, not at all.
But if you want to focus on industry, let's talk about FS canon.
By FreeSpace canon the colonies lost the bulk of their industrial base with Sol. Since then they have splintered, reunified, beaten off a second Shivan incursion, lost a major system and slumped into a depression.
Sol, which had the bulk of all human industry at the time of the Isolation, went through a collapse and then a boom. It is a solar system.
Its sum total population could not begin to come close to denting all the resources present in it. The asteroid belt alone could feed the industrialization of Sol for centuries to come.
Given that Capella was a major system with only a few million inhabitants, there is no way that the summed industry of the colonies matches Sol, even - perhaps even especially - fifty years later.
Again, this is all by FreeSpace canon and extrapolation thereof.
But it's not matter of industry alone. Sol is a symbol.
Did you read the previous posts aimed at you? Did you see the points made in the previous one enumerating the reasons for the invasion? Economics are one driving force but not the only one.