This is NOT the case for this story. They explicitly said that the moment Shinpei returns, a new timeline is created, but the old timeline is also erased. Meaning, there is only 1 actual timeline, not multiple parallel universes always existing at the same time.
EDIT: This is the actual explanation in episode 21:
Shinpei's power is the ability to observe parallel universes. When he dies, the world is duplicated many times, and only his consciousness can travel to any one of those worlds. Once he does (the part where he wakes up in the "past"), everything that happened in the world he was previously in is gone, as if it never existed. So there are no failure timelines left hanging around.
Episode 23.