Agreed.
What you wind up with amounts to "layers of archipelago" that somewhat naturally creates a hub and spoke network at the upper end of high volume interstellar trade to high population worlds ... down to more of a point to point tramp free trader "local trader" serving the lower end of the low volume trade to the mid and low population worlds.
This then naturally forms "trade volume brackets" (high/mid/low) which then in turn dictates the displacement of starships capable of carrying those volumes in ways that are ultimately profitable (for someone, somewhere). Whether the profits come from ticket revenues for transport services or from the arbitrage of (speculative) goods, merchants need to make profits in order to remain in business (profit or perish, basically).
One of the ironies of Traveller is that there isn't much of attention paid to the migrations of world populations. Pretty much every world on the map just stays "stuck" at whatever UWP population number got rolled for it. Things like boom & bust cycles prompting waves of migration have to be inferred, based on the distribution of population numbers around a subsector/sector.
Active colonization efforts can actually raise a UWP population number pretty quickly.
- Population: 0 = 0-9 people as permanent residents
- Population: 1 = 10-99 people as permanent residents
- Population: 2 = 100-999 people as permanent residents
- Population: 3 = 1000-9999 people as permanent residents
- Population: 4 = 10,000-99,999 people as permanent residents
As you can see, all it takes to raise a population from 0 to 1 is the delivery of "a couple free traders worth of passenger settlers" and a world gets a population bump of +1.
Going from population 1 to 2 might take another "dozen or so free traders worth of passenger settlers" to get another population bump of +1.
After that, the rate of growth will necessarily slow down unless you get more ships involved to transport colonists/settlers ... but the point still stands. At the low end of the scale, it doesn't take a huge number of immigrants who stay permanently to increase a world population. However, those people aren't going to "pull up stakes" and move someplace with no opportunities for them to make a living, so being able to "jump start" a world economy and attract enough people to climb out of the Population: 0-4 "doldrums" becomes something of an interesting challenge. The hard part is avoiding
Dutch Disease while founding a world's economy, especially since almost all Non-industrial worlds get founded on a basis of resource extraction for interstellar trade. Whether worlds can "grow beyond" just being resource extraction nodes in the broader trade network then becomes the challenge.
Agricultural trade coded worlds are perhaps the easiest to "jump start" since they only require a minimum population of 100,000 people. In groups of 8 passengers, that amounts to 12,500 passenger blocks of 8 people each to reach that minimum number (and ignoring birth rates and death rates). That may sound like a daunting number of passengers for a single starship to transport ... but if you broaden the time horizon from "instantly" out into decades (3-4?) and use multiple starships for the task ... things start looking a bit more doable.
A fleet of
Type-M Subsidized Liners transporting 20 colonists per ship can move ~240 people "one way" (outbound to a colony) each year.
A fleet of 11 such
Type-M Subsidized Liners chartered to operate for 40 years can move
105,600 people to a colony world within 3 parsecs of the world exporting colonists. That's enough to establish an Agricultural trade coded world if the birth/death rate plus emigration rate away from the colony world adds up to a net zero loss of population during those 40 years.
On a human scale, that would be a long term project.
For a representative government that needs to get periodically re-elected, it is definitely a long term project.
For a corporation, technocracy or civil service government, it's a long term project ... but one that can be accommodated within their long term planning budgets.
My point being that such projects are possible ... but they require a tremendous investment of time, capital, political will(ingness) and political continuity of purpose to accomplish. There will always be opportunities to derail such a colonization project, for reasons various, sundry and parochial ... but it CAN BE DONE if there is sufficient investment in the prospect.