I don't think it's "finally some hope" or anything like that. The author has been building up to this for a while. As much as everything keeps crumbling around Reiji, and he pretty much had more than half the people in his life willing to commit a lover's suicide with him, he's never once actually come close to making that decision. He could've laid on the ground with Nagi to slowly bleed out. Nobody was around for miles probably, yet he makes the decision to save her, whether or not he understood what he was doing. He was depressed all the while, but for how close he was to the bridge at all times, he never made the plunge. All he needed to do to realize what he really wanted was an eventual epiphany from an outside source.
Reiji finally understands now why anything in his life was happening the way it was. He sees his mother pulling everybody into the abyss, and after finally being able to look at this from a far off perspective, he makes the decision to pull everybody out of the abyss instead as the only person uniquely positioned to be able to do it. It only took the outsider perspectives of the writer and Nagi for him to realize just how brainwashed and manipulated everybody around him was.
I think psychologically, it's just a really fucking well written piece. I think there are presences in real life where you don't really realize they're incredibly bad influence on themselves and the people around them. It takes somebody from the outside to kinda break your reality and see things for what they actually are, then the healing can eventually begin. This is probably the slow and painful healing process to breaking the town away from his mother's influence.