While its certainly true and canon that the sector is spiraling down into further and further collapse, I feel it is also right that they did have to do some stabilizing, as nations form, borders and alliances are made and set, and so on, rather than the 'free for all' starving madness that is said to have happened in the following decades to the collapse.
So while there's a constant loss of knowledge, at least populations dont starve and fleets are in place to protect and such.
And given how Jangala is praised to hell and back for being nice and habitable, I would imagine slightly making one of your worlds more habitable would give you a rather big strategic advantage.
But even if we accept the idea that the factions are far too busy fighting and posturing, what has stopped another freelancer like us or minor faction/group from trying?
Asharu is independent, they dont have horses in the posturing race, so they could surely carry out tugs and such minor projects without much help or interference.
Nevermind that solar shades and mirrors are ridiculously primitive and low tech things to make compared to spaceships. And thatss dicounting nanoforges and all that super tech they can get.
Again, not saying they can turn worlds into new earths, thats clearly a more dedicated job and with Eridani-Utopia off the sector, a no go.
But fixing planets being frozen or scorching hot is relatively speaking, peanuts for a space infrastructure owning planet.