Such a mechanic would need to have a longer RoI than building outward and/or compete with outward expansion in some way (like costing influence).I find that this is an ephemeral and moving target and I'm not convinced the target has ever actually existed anywhere and in practice has always been small because it describes a false dichotomy.
And see, I want the same thing in the game for a different reason - I want finding habitable worlds, ones with independently stable biospheres, to be significant and multiple viable planets in one system should be an outright unicorn. I also want to be seeking out planets that aren't usable now, but could be made usable later. I also absolutely intend to play as wide as possible within such parameters.
If that were the case, you simply could not build both wide and tall at the same time (but because of opportunity cost, rather than hard constraints).