Megastructures are too expensive for tall empires. They might have the influence but the resources simply aren't there. Square pegs don't fit very well into round holes, not even with special rules. Habitats are much more useful for building up tall since they can maximize the power of a small territory. However they aren't the magic tall pill either because habitats also benefit whoever has the most resources to throw into them.The point of that limit is to restrict wide empires. Your suggestion contradicts the initial reasoning.
Playing tall only makes sense if you can conquer the map without necessarily having to personally paint the map. The most appropriate mechanics for tall play is from building alliances, boosting trade and gaining influence over empires. If those options are too weak, then tall ends up being too weak. There is no megastructure that helps you boost trade, build alliances or conquer empires. There is no megastructure that requires you build an alliance, get help from other factions or be a galactic beacon for anything but yourself. Habitats aren't too bad, but everything under megastructures and their benefits are absolutely biased towards making wide empires better. It's pretty safe to say that megastructures just don't accomplish their goal of going tall and it's pretty silly to think they should, at least as they are currently.
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