How would they have achieved the levels needed for full independence without the population? They don't even have a colonial government until they become a full colony.
It'd be like jumping an unorganized territory to statehood without ever forming a territorial government first.
The proposal I include as well as Aelfric's proposal have local governments included. But in the case of my own proposal they are jumped to the phase of full colonization that
I see this as unnecessary. Self interest would keep the colonizer spending money on their colonies.
You are presuming that rational self-interest will continue unabated in the universe of Warhammer 40k. Frankly I don't think that's a good presumption. Sure, the Nine Worlds as they're ruled now are pretty competent, but we can't presume that there won't be a few corrupt governors in later colonies who won't do stupid shit.
Frankly, I find it unlikely that resource colonies will have long term residents. It's more likely that young men will go there to get higher paying jobs, save their money, and then travel back to their home worlds. That's been the traditional model for resource extraction - the gold fields, the oil fields, etc.
I'd also point out that making the resource colony a direct extension of the home world's government will greatly facilitate this.
The full colony cutoff is for the exceptional situation where a resource colony has for whatever reason grown out of being just a resource colony. That isn't likely to happen. If it were, you'd just found it as a full colony in the first place.
That's the whole point of resource colonies. They are a way to exploit the resources of a system that is not viable as a full colony.
I think using gold and oil fields as a model here is a mistake, because they don't scale to an interplanetary model where you're going to have millions of workers of both sexes. There will be people who are having families in these colonies, and children who will be growing up there and eventually working there. Look at Niflheim and Muspelheim before the collapse of the Imperium - that's the closer to the model resource colonies are likely to operate on. This seems to be more along the lines that the High Councilors are thinking, given their proposals. Resource colonies will be smaller and on less desirable worlds, but it's rather likely that many of them will grow to the point that they get half a billion people at some point - see how Niflheim now has 887 million people now, up from a significantly smaller number.