WARNING: The following contains spoilers for Demon Slayer The Movie: Mugen Train, in select theaters now.
Classic tropes and concepts are frequently recycled from one generation to the next. Expert storytellers will honor the core elements of these archetypes while adding new and fresh twists to them to create something they can call their own. This happens routinely in the world of shonen anime/manga, and clear parallels can be drawn between Demon Slayer and the venerated Bleach.
Tite Kubo's Bleach was one of the "big three" of its time, and some of today's shonen fans informally consider Demon Slayer to be part of the new big three. However, while Bleach may stand as one of the best out there, these two series share many elements. Namely, their mutual "monster hunter" theme that Demon Slayer arguably takes a more balanced approach with, using the mortality of the titular Demon Slayers to give horror stakes to the action series.
Demon Slayers Find Strength In Vulnerability
Characters with flaws, weaknesses and vulnerabilities give way for growth and improvement and add tension to the narrative by having those fragilities. In shonen, those weaknesses often appear when a hero is pitted against increasingly dangerous enemies, frequently prompting the protagonist to take their abilities to the next level. However, while Bleach and Demon Slayer both do this in roughly similar ways, Demon Slayer arguably does it better, and in more than one way.
A significant difference is the very nature of Demon Slayers vs. Soul Reapers. Demon Slayers are forever the underdogs, being mortal humans who are facing off against supernatural Demons. The lore makes it clear that human strength can never match demonic force, so Demon Slayers train hard to use breathing techniques to boost their power, and they also use Nichirin blades forged from special ores to slay Demons. Without those, Demon Slayers are nearly as helpless as the innocent people they defend, and there is no changing that fact.
This vulnerability adds massive amounts of tension, and even the Hashira -- the highest-ranking Demon Slayers -- may succumb to powerful Demons, such as the Upper Moons. At the end of Mugen Train, for example, the Flame Hashira Kyojuro Rengoku put up a terrific fight against Akaza, the Upper Moon 3, but even he couldn't survive. Still, Rengoku's conviction and determination meant he ultimately won the battle by keeping everyone safe. Many of Demon Slayer's battles come down to these hard-earned victories where the underdog Demon Slayers defeat their supernatural foes.
Bleach's Powerful Soul Reapers
This is not to say that Bleach's combat system is broken or tension-free. After all, Bleach is renowned for its many challenging and exciting battle scenes, from Ichigo's final duel with Byakuya Kuchiki in the Soul Society arc to his and Grimmjow's grudge match in the Hueco Mundo arc. However, Soul Reapers feel like a product of their time, putting a little more emphasis on dazzling spectacles than vulnerability. Looking back at Soul Reapers like Byakuya, Toshiro and even Rukia, they have some vulnerabilities but feel like vehicles for exciting fight scenes.
Soul Reapers aren't even human, in fact; they are souls -- powerful spiritual beings that use specialized soul-cutting swords called zanpakuto to carry out their duty. It takes a lot of training and experience to be an effective Soul Reaper, and they are powerful enough to slay regular Hollows with utter ease. Thus, the story of Bleach quickly moves on from regular Hollows to antagonists such as the mighty Espadas and later, the Sternritter as well.
It's fun to see the Soul Reapers fight those foes, but rarely are they genuinely sympathetic in the process. Even Ichigo, who has a human perspective, started from zero and quickly became a demigod on the battlefield. For some, the vulnerability and mortality of the Demon Slayers could be more engaging than the Soul Reapers' "best of the best" status. While each series has strengths and the love of their respective fans, Demon Slayer thrives with its horror-like survival theme that pits humans against the much stronger Demons.