To all those saying that characters being revived ruins the tension and such: maybe revisit the story. If you think the revival of some of the dead characters makes their past deaths inconsequential, then it's you who are reducing the meaning of their deaths not the author. The death of all the characters here (Tonari, Gugu, March etc.) was sad for more reasons than "they are no longer breathing".
Throughout the story there is a continuous emphasis on the bittersweet effect of time. Fushi is immortal, he's constantly leaving friends and loved ones behind just by out living them. As the story got on, time skips gradually got longer and more frequent. It goes from a few days (March), to a few years (Gugu) to a few decades and generations (following the island arc with Tonari). This isn't a "just because" detail of the author's story telling, this is the author attempting to convey Fushi's life from his perspective.
The longer he lives, the more weary and tired he becomes and the more he drifts from his humanity as a consequence. The humanity he's specifically gained as a result of the bonds he's built with the people who have died. This whole arc has been a culmination of over a century of Fushi slowly being worn thin. He's become stronger as an immortal, but despite not being human - he has a very human heart (despite how much he's tried to ignore it) and time and loss has taken a toll.
After over a 100 chapters, Fushi is finally being reunited with close friends who shaped the person he is today. People who were all taken unfairly too early from him because of Knockers. Only Pioran could be said to have died peacefully of old age, and even then her last moments were rendered bitter by the mental illness brought on by her old age. She came back, but not as she once was and Fushi's final memories of her will always be tinged with the memories of her mental deterioration.
This is the moment of catharsis we have been waiting for, the first fair hand he's been handed to him in all the years of his long, long life. Their revival doesn't erase the years of pain, mourning and guilt Fushi experienced following each subsequent death of his friends. It doesn't erase or undermine the emotions or stakes resulting from their death within the story.
March still endured a death no child should. She still lost the chance to live with her family.
Parona still lost March, a failure reminiscent of loosing her sister. She died a dogs death offscreen, beheaded and mutilated by her sworn enemy.
Gugu died on the cusp of adulthood, just as his relationship with Rean was flourishing and Rean had to live the rest of her life forsaking the idea of falling in love again.
Tonari lost her close friends at an early age and still had to live on until her 50s before dying from Knockers.
The nameless boy - the first human form Fushi took - died alone, with his dog who wasn't even truly his dog, following a failed expedition where he learns that all the people who left him for better lands died unbeknownst to him, and that no one was ever going to have returned to him.
None of the above has changed. And even if every single character was revived, it would not make their death any less consequential. They still suffered, Fushi still suffered.
It's just now, after all that suffering, we're finally seeing the end of the tunnel. Its finally the dawn to a long, drawn out and restless night.