[randomname93857]

>> Furthermore matter is just condensed energy, so why bother with some wonky immobile megastructure when you can just decay your everyday trash into pure energy.

True, matter is an energy... however the only known way to convert matter into pure energy is to make it annihilate with antimatter ( akin to electron-positron). We first have to have a good and cheap source of antimatter! And the matter annihilation may still generate a lot of particles/matter - that may be a waste (or not) depending what you do with that further.

A Partial conversion of a matter into energy is also fusion and fission reactions, but they are limited by the difference between source and end element of reaction here https://en.wikipedia.org/wiki/Iron_peak#/media/File:Binding_... - e.g. H2 -> He3, That uses only a tiny portion of matter, no energy that can be extracted from Fe56 (max), and all its close elements are also useless for either fusion or fission energy extraction.