But where does that money to invest come from if 90% of the population doesn't need to work? Inheritance from previous generations that did work?
Perhaps; or maybe every child is (at birth) gifted a small portfolio by the government; or their parents; or a charity; or a random philanthropist. That small portfolio grows as the child does.
Without changing the complete TL charts going back decades, I am not sure how to change that. Nor am I sure why wouldn't want to. Not in Charted Space anyhow. You'd have to completely rewrite the Ancients as well.
I do not recall seeing 'Post Scarcity' on a bunch of TL charts; but a brief skim through previous editions suggests that 'endpoint' technologies were placed at the high end of the Technology scale. The number of Technology Levels has risen over time from 18 in LBB #3, to 35 in T5; but the implication has always been that 'perfection' was not available, that there was always more progress to be made in every field even at the next-to-highest TL.
Anyhow; it looks like you and I are not likely to agree on this right away, so let us set it aside in favor of other things which we might be able to agree on.
Right now it looks like we both agree on: materials produced by prospecting -> (Mineral Refinery) -> Ore -> (Smelter) -> Raw Materials -> (Manufacturing Plant) -> Stuff
And I like the flow of (Raw Resource) -> (Processed Resource) -> (Commodity Items, Specific Components, & etc) -> (Finished Goods) -- but I think it is inconsistently applied in Traveller.
And this works fine for TL 7+, where Manufacturing Plants actually exist; but it leaves TL 6- worlds in limbo. Also, I would like to examine how:
1} a world builds the FIRST Manufacturing Plant;
2} how a planet upgrades Technology levels;
3} get a better grasp of how non-metal stuff is handled; and
4} the distinction between 'Advanced Goods' and 'High Tech Level Goods' needs to be examined & clarified.
For 1} can we adapt the rules for 'worlds without shipyards building ships'? I seem to recall seeing those somewhere, but I may be misremembering.
I think if we are going to have (Manufacturing Plant) -> Specific Components -> (Manufacturing Plant) -> Specific Finished Goods, then maybe we can specify that there are some classes of Manufacturing Plants of special interest. Some Examples:
A} A 'seed' Basic Manufacturing Plant (BMP) which can be built without specific components -- but all it does is manufacture specific components to build other BMPs.
B} A BMP which just produces the initial components to build a 'seed' Advanced Manufacturing Plant (AdMP) which itself only produces specific components to build other AdMPs.
C} A 'seed' Agricultural Manufacturing Plant (AgMp), made from components from a specific BMP.
D} A 'seed' Specialist Manufacturing Plant (SMP), made from components from a specific AdMP.