. What is the point in keeping a high pop in GCIII?
Production, production, production.
The production system is radically different in GC III than it was in GC II.
In GC II most of it was down to what buildings/starbase modules one has affecting a planet and the galactic population one possessed (via the sliders).
However, in GC III production is greatly influenced by the population of the planet itself and not the galactic population.
It's the difference between assigning galactic tax revenues to each planet equally and depending on local 'revenues' instead. In GC III each planet has to depend on itself and not the galactic whole. Well, outside of galaxy wide techs and starbases and the like.
To put it another way, in GC II, a new colony that started with 500 million people and a new colony that started with 5 billion people had the exact same production upon settlement (presuming everything else was equal). That's because it was depending on galactic revenues. However, in GC III, that 5 billion planet will produce things faster than the one with 500 million starting out (again, presuming everything else was equal).
Others have looked into this far more than I, but according to the devs, the "sweet spot" is around 45-50 billion people per planet. After that, the law of diminishing returns comes into play and one should instead of building more farms, should instead focus on buildings that do manufacturing/research/wealth/whatever.
I am still mastering it so I can't comment on it fully, but while one will probably want to build production stuff to kick some planets into gear, getting its population up ASAP (relatively) will help greatly in all areas.
And that's not even getting into "perfect play"/micromanagement hell (which I refuse to engage in, I will hasten to add
). Even if one just cranks up the population to 40 billion or so and then slacks on building the 'perfect' placement of buildings/starbases, one will still be doing far better than if one left planets in the low population zone.
Population and the Production Wheel being set per planet is probably the single biggest and most radical change between GC II and GC III. And one that isn't apparent at first glance. Especially since one isn't limited to 16-20 billion people per planet anymore.