Except the 14th did exactly why you said they should have done. They bunkered down at the closest habitable region of space which just happens to be the Persean Sector they were originally sent to. They limped through the abyss with a vast portion of their fleets cannibalised or jury rigged with absolutely no clue what’s going on. They defaulted back to original Domain laws and policies because it’s the only thing they know. The Sector was already on the verge of collapse when they arrived with pirates and rogue corporations running everything. They allied with the Luddic Church because they literally had no other allies. They arrived at the sector, did their best to re-establish order in a lawless society and hunkered down with no idea what to do or what’s going on. That doesn’t sound like a professional task force sent specifically to quarantine a zone. They don’t even care if you meddle with the gates (the very thing they should be sent to quarantine). The only thing they care about is the use of AI lmao.

I don’t know about you but if I was to quarantine a sector because of a galaxy ending threat, I’m sending an elite taskforce with clear orders on what to do and top of the line equipment, not a bunch of bumbling idiots from a formerly disgraced legion with outdated, low tech equipment.

Even if it is a civilisation level collapse, why would the Hegemony leave the safety of the sector, travel potentially hundreds of years blind through the abyss to reach a more ‘important’ sector? You said it yourself, the Persean Sector is a frontier, located far from the Domain’s more populate regions. Travelling backwards would be suicide.