The Shichibukai, also known as the Seven Warlords of the Sea, were a group of elite pirates that were affiliated with the Marines. It was created by the Marines to repress the threat of pirates by recruiting other notable pirates to work for them. Joining the Shichibukai came with many perks, which is why many pirates actively tried to find a way to join the group. The pirates in this group were allowed to get away with all kinds of activities without being questioned, and they weren't even held accountable by the Marines.
When Luffy started defeating the Shichibukai, their secrets were exposed to the public. The rulers who had suffered because of Crocodile and Doflamingo made it a point to bring up the abolishment of the system at the Levely. While the Straw Hats were in Wano, the world was graced by the news of the Shichibukai system's dissolution. The decision to abolish the system was also affected by the Special Science Group's project— the Seraphim. When it was first revealed that SSG was working on a project, nobody expected the result to be so impressive. After seeing the Seraphim in action, many questions were raised about whether they would successfully replace the Shichibukai in One Piece.
The Strength of The Shichibukai
The first-ever member of the Shichibukai to be introduced in the series was Dracule Mihawk. As the story progressed, the other Shichibukai were also revealed. The Shichibukai had varying levels of strength, and while some of them were more powerful than the others, all of them were strong in their own right, except for Buggy. The likes of Mihawk and Blackbeard could fight against the strongest characters in the series while Buggy and Crocodile were defeated by Luffy, before he even knew to use Haki.
The strongest Shichibukai was unquestionably Dracule Mihawk, followed by Blackbeard and Boa Hancock. Each member of the Shichibukai had enough strength to take down many strong fighters. This made them an invaluable asset to the Marines, which is a key reason why they were free to do whatever they wanted to.
​​​​​​In addition to their ridiculous strength, the different members of the Shichibukai helped to progress the plot of the series. The Straw Hats defeated many Shichibukai, which included the likes of Crocodile, Moria, and Doflamingo, as they served as antagonists in many arcs. Other members such as Jimbei, Trafalgar Law, and Boa Hancock have helped the protagonists to achieve their goals.
The Genetic Monsters
The Seraphim have been created by the Special Science Group. They are superior to the original Pacifista that made their debut in the Sabaody Archipelago arc. The Seraphim have been created through the use of the Lineage Factor of several characters. Unlike the previous Pacifista, who were modeled after Bartholomew Kuma, the Seraphim have different designs. So far, a total of four Seraphim have been revealed, and each of them resembles the younger version of the Shichibukai. The Seraphim have black wings and white hair; these traits are unique to the nearly extinct, Lunarians.
The Seraphim units have a great deal of strength, as shown when they fought at Amazon Lily. The Seraphim of Dracule Mihawk was able to slice off a chunk of the mountain. It was even able to fight against the Blackbeard Pirates and Kuja Pirates without any problems. However, its biggest accomplishment was pushing Blackbeard on the defensive, who acknowledged that it was nothing short of a monster. The Seraphim that fought on Amazon Lily were unscathed, which shows that they are extremely durable.
The Straw Hats came across a Seraphim for the first time on Egghead Island. The Seraphim was given the name "S-Shark," and it looked like a younger version of Jimbei. What's surprising is that it could perfectly use Fish-Man karate against its opponent. To make matters worse, it also had the powers of the Sui Sui no Mi, which is a devil fruit that belonged to Senor Pink. Currently, it is unknown whether the other Seraphim also have devil fruit abilities.
Have They Replaced The Shichibukai?
The short answer is yes. Absolutely. The Seraphim are far superior in strength to most Shichibukai with a few exceptions. According to Vegapunk, they continue to grow after every fight, which makes them even more lethal. The best thing about them is their lack of will. Due to their inability to act on their own, it is easy to control them. The Seraphim are programmed to obey orders, and they do so without fail. They can carry out the most dangerous missions, and chances of failure are very low.
Furthermore, the Marines do not have to grant them special amnesties to make them work for them. The advantages that the Seraphim give the Marines are endless. The Seraphim will mainly serve as loyal servants to eradicate any threats to the World Government's authority. So far, they have been deployed to capture former Shichibukai, but there are still some Seraphim that are yet to appear in the series. Considering the Shichibukai had seven members, it is safe to assume that there should be at least three more Seraphim.
In terms of impact on the story, it is impossible for the Seraphim to displace the Shichibukai, but purely based on strength, they are far superior to the pirates. Another thing to keep in mind is the fact that Vegapunk intends to run away from Egghead Island, so will he recall all the Seraphim, or has the World Government prepared countermeasures already. Along with these questions, there are other mysteries surrounding the Seraphim, which will be revealed in the Egghead Island arc. While Vegapunk has mainly focused on the ancient kingdom and Ohara, fans can expect him to go over the secrets of the Seraphim project in the future.