Judging by how incredibly advanced domain tech actually was it's fairly believable to say that the sector is a backwater compared to what it was before. The [Ultra-redacted] stuff implies that the domain wasn't just a vast spacefaring civilisation, they were running around with weapons that breaks the laws of physics, and the gates and their planets were fueled by the hypershunts (which harvest energy directly from stars and then transport said energy light-years away). Compared to that, fusion reactors and shells are very antiquated.
And the vast fleets of space ships demand nanoforges to be built, but as far I can tell no one actually knows how to make more, and as such the sector is definitely in a downward spiral, because the current technology base has an expiration date. The sector will keep dwindling as long as they cannot make new nanoforges, because the forges are the backbone of all their industry. Without the nanoforges the sector can only produce d-modded garbage, and the supply of heavy machinery, supplies, heavy weaponry and ship hulls will dwindle away to nearly nothing (as even a damaged nanoforge increases the output by one unit, which is implied to be a factor ten for every unit, so a world with a pristine nanoforge produces a thousand times more ships than a world without it).
I don't think humanity will disappear from the sector entirely, but right now one of the most important part of the economy, the manufacturing, is slowly disappearing and as such the sector is in a decline until either someone rediscovers how to create nanoforges or the sector loses all their nanoforges and has to rebuild their civilisation without them.