Although the Rinnegan could bring the dead back to life, the price was extremely steep: the user had to sacrifice their own life. Its side effects were fully comparable to those of the Reaper Death Seal. Both ultimately ended in death.
The difference was that after using the Reaper Death Seal, the user's soul entered the Grim Reaper's stomach. After using the Rinnegan's Resurrection Technique-Outer Path: Rinne Rebirth-the user's soul entered the Pure Land, just as it would after an ordinary death.
In the original story, the Six Paths of Pain invaded Konoha and caused widespread deaths throughout the village. Even Kakashi and Shizune died.
However, they had only been dead for a short time, still had bodies capable of housing their souls, and had their souls collected by the Naraka Path after death. That was why Nagato could resurrect so many people when he used Outer Path: Rinne Rebirth.
At the end, however, when Madara Uchiha was resurrected, only one person was brought back.
Still, whether it resurrected one person or many, the user of Rinne Rebirth ultimately paid with their life.
Obito Uchiha alone had briefly reached the Six Paths level, so he remained alive and kicking for a while after using Rinne Rebirth.
Really, Masashi Kishimoto had simply given Obito Uchiha cheats, stacking his bugs to the limit.
First, he fell from the Six Paths level. After becoming the Ten-Tails Jinchuriki, he had the Ten-Tails extracted from his body. At that point, Obito Uchiha had already met a condition that should have guaranteed his death.
Then Black Zetsu made use of what was left, possessing Obito Uchiha and using his body to perform Rinne Rebirth and resurrect Madara Uchiha. Add another death sentence.
Finally, he joined Kakashi, Naruto, Sasuke, and Sakura in fighting the Big Boss-Kaguya Ōtsutsuki-and