Bleach is a hugely popular shonen action series that has greatly expanded its combat system over time in order to deepen the lore and excite viewers with newer, flashier ways to fight. In the early episodes/chapters, protagonist Ichigo Kurosaki battled basic Hollows with a generic zanpakuto with no abilities, but that all changed in the legendary "Soul Society" arc.
This arc significantly expanded Bleach's combat system by adding more Kido spells, elite martial arts, and, most importantly, both shikai and bankai as special zanpakuto releases. Bankai, in particular, is a Bleach staple that frequently wins any battle for the heroes, but bankai has now become a problem, and the Bleach franchise appears hesitant to fix it.
Subsequent Arcs of Bleach Retconned & Cheapened Bankai
The "Soul Society" arc introduced the concept of bankai, mostly through the aristocratic Captain Byakuya Kuchiki. The way Byakuya explained it, bankai is a rare and incredible power that only one person every few generations can unlock, and everyone who ever obtained bankai had their name recorded in the annals of Soul Society history. Bankai was set up to be a rare, monumental thing that only society's elite could even dream of having, and Byakuya certainly had one. He was born into the noble Kuchiki family and had immense natural talent and a strong lineage, so bankai was on-theme for him. Too soon, though, Bleach disregarded this bit of lore and immediately started using bankai as a fun but cheap way to power up its heroes in otherwise unwinnable fights.
Even in the "Soul Society" arc itself, Bleach retconned its bankai lore twice. First, Lieutenant Renji Abarai unlocked bankai so he could fight more evenly against Byakuya when they battled over Rukia's fate. Then, Ichigo did the impossible and unlocked his own bankai, Tensa Zangetsu, in a mere three days. Even for noble-born Soul Reapers like Byakuya, that is ludicrous, let alone for a human like Ichigo. What's more, Ichigo got help from Yoruichi and trained with a contrived practice dummy designed to help people like him achieve bankai with extreme speed or die trying.
Even before Byakuya revealed his bankai, Senbonzakura Kageyoshi, bankai became a convenient narrative tool that had no relation to its mystical lore about historical, one-in-a-generation talent. Bleach then kept this up in the "Arrancar" saga, with Ikkaku Madarame unveiling his melee-type bankai purely so his difficult duel with Edorad Leones could become winnable. Finally, Bleach gave Rukia herself an icy bankai, Hakka no Togame, merely so she could finally defeat As Nodt and avenge Byakuya's earlier defeat against the Sternritter. While many Bleach fans loved seeing these stylish power-ups, it also cheapened the concept of bankai, turning it into a last-minute gift for any hero who needed it to win a fight. It was an artificial and contrived form of power scaling -- something for which Bleach is now notorious.
How Bleach Too Much Relied on Bankai
Even by shonen standards, where a few contrived power-ups are forgivable, Bleach got carried away with its contrived efforts to power up its heroes, and bankai isn't the only offender. The series made similar missteps with power-ups, such as revealing at the last second that Yumichika Ayasegawa's shikai was a kido type rather than a melee type. Later in the Bleach manga, Toshiro Hitsugaya obtained a stronger adult form that wasn't properly set up or foreshadowed and hasn't been used again since. Similarly, Captain Sajin Komamura spoke to his reclusive clan elder to obtain a one-time-use power to gain human form so he could defeat Bambietta Basterbine in their second duel. Then Sajin shrank into a helpless dog, never to fight again, all because Bleach wanted him to win against the Sternritter who stole his bankai.
Bleach could and should have used other well-established shonen strategies to help its heroes overcome impossible odds in combat and win the day. Rival series such as Naruto, One Piece, My Hero Academia and JoJo's Bizarre Adventure all do this, with JoJo and Naruto in particular making great use of creative but non-contrived ways for the heroes to win. It's true that nearly all shonen series sometimes resort to contrived abilities or twists, but even then, Bleach could stand to emulate those other series a little more and give bankai a break.
In Naruto and JoJo, the heroes often make clever use of their abilities, think in resourceful ways and use combo strikes to win the day. It's impressive how often JoJo heroes win without needing last-minute power-ups, simply using their Stand and the battlefield itself to exploit the enemy's weaknesses and win. This makes the victories far more satisfying and less predictable, and even My Hero Academia often does this with its Quirk-based combat system. Bleach could do the same with non-contrived strategies, such as making better use of the terrain and using advanced kido spells, shikai and teamwork-based strategies.
Bleach's combat system is deep and has several compartments that could, in theory, allow for some truly spectacular strategies and victories that don't rely on generous, lore-breaking use of bankai to help the heroes win. Ichigo and Rukia are both tough, clever and self-reliant punks who can look after themselves, like Josuke Higashikata and Naruto Uzumaki. It would therefore be very on-theme for them to use everything but bankai to pull off upset wins against the likes of As Nodt, Byakuya Kuchiki and more.