The isekai genre is one of the biggest in manga, anime and light novels, and its ubiquity is also felt in Korean manhwa and Chinese manhua. The genre featuring protagonists being reborn into new lives has also developed both its own list of tropes and a recent propensity for the increasingly absurd.
Perhaps due to how blase the genre has become, many now bemoan each entry's trite beginnings in the "other life" before the characters are reborn. While cutting out narrative fat makes sense, isekai as a genre still needs the original life to some extent, if for no other reason than thematic contrasts. Here's why the genre's biggest trope needs to stay around, warts and all.
That Time Before I Got Reincarnated
In most isekai, particularly of the popular male power fantasy variety, the protagonist starts their first life as a relative "loser." They may be a shut-in or otaku and typically have few friends, escaping away by spending their days in online video games or other such activities. They may be either ignored at school and work or outright picked on, making them a lovable character to an audience that may very well relate to. After all, feelings of loneliness are felt throughout all walks of life.
The protagonist will then be sucked into, or in some cases die, before being reborn into a new life. This reincarnation is usually far more colorful and bubbly compared to their mundane and put upon previous life. They may find themselves able to access incredible powers and meet creatures of all shapes and sizes, quickly embarking upon a far more adventurous and rewarding life.
Recent inverses may have a protagonist with a relatively normal life, only to be reborn into a far less pleasurable one. They may still be able to form some semblance of their old life or at least a facsimile of it, but many times, any semblance of normalcy is completely gone. This has upended the power fantasy trope of the genre and given it far more diversity. Still, it also helps to highlight why focusing briefly on the character's previous life is so vital since the theme of change is often an important one.
The Second Life Needs to Contrast the First One
Having the original life presented before the reincarnation occurs or shown concurrently in flashbacks, isekai series can illustrate how much better, or sometimes worse, this new life is. For people who were looked over or picked on in their previous life, becoming a powerful warrior or some other such popular figure in their new world is a dream come true. They suddenly have the power, influence and popularity which they had always desired in their old life, and through it, a way to defend those who are as helpless as they once were.
Likewise, someone with a far more enviable previous life will be particularly distraught when it's upended for a much less luxurious one. When all semblance of normalcy and humanity is wiped away, and they're forced to rely on others for mere locomotion, the story in many ways becomes a psychological and body horror instead of a fantastic adventure. Similarly, the chance to possibly return to their old life could be a driving force, but that old life only becomes a desirable goal if what made it so much better is relayed to the audience.
While audiences may tire of introductory episodes of initial lives if they're boring or poorly done, the concept is still a thematic necessity to the ever-popular isekai genre, especially as it continues to try to innovate.
