Tsugikuni Yoriichi had accepted Tesoro—or rather, Tesoro possessed exactly the kind of ability Yoriichi lacked in developing the New Marines.
In terms of strength, Tsugikuni Yoriichi was undoubtedly one of the most formidable people on these seas. But a person's energy was limited, after all, and there were many things Yoriichi was not good at.
Moreover, none of his companions possessed exceptional managerial talent. Issho was upright, but he was blind, and the affairs of the New Marines were usually handled in part by the adjutants under his command.
Kyros was fairly versatile and had received a formal, excellent education at Marine Headquarters. However, he was now the son-in-law of Dressrosa's royal family and had a child. He could not possibly devote his energy to the New Marines' affairs.
Shimotsuki Koushirou had also returned to family life. Most of his time was spent helping Tsugikuni Yoriichi improve the Marine Academy's talent-training system.
As for Mihawk, Abel, Kuma, and the others, Mihawk only wanted to defeat Tsugikuni Yoriichi and become the world's greatest Great Swordsman. He had no desire whatsoever to involve himself in Marine affairs. Abel and Kuma, meanwhile, had rather sensitive identities, limited educational backgrounds, and limited management abilities.
As for Fisher Tiger and Jinbe, their Fish-Man identities also restrained them to some extent. Simply stabilizing the foundations of Risky Red Island and Fish-Man Island had already consumed all their energy.
In recent years, the New Marines' development had slowed, and their sphere of influence had been unable to expand further. The main reason was the lack of management talent.
The Marine Academy had indeed trained some people in recent years, but they still needed time before they could truly grow into figures capable of standing on their own.
Tesoro's addition could inject new strength into the New Marines.