Yes kame couldn't keep that ship afloat the drug scandals from taguchi and koki really tanked things. But the early big blow was jin leaving. He and kame are the best vocalist and song writers in the group. Plus jin was the leader and suddenly it all was now kame's responsibility. Ueda just works lol you don't get much trouble from him but the issue with him and Maru is they weren't as popular and their skill set now as robust as the top two members of kat-tun. Kame was doing movies TV shows MC and dramas and had to carry solos and group tours by himself. I'm not surprised that he's tired now. I still remember kat-tun setting the fashion trend almost every outfit they wore was cool af and this was when stylists actually had a real job and made custom wardrobes instead of just throwing designer brand trash at the idols. Japan in general when it comes to the entertainment industry and some cultural issues is what truly prevented jpop from being what kpop is today
Jin was never the leader of anything (except maybe scandal-making). In fact, the group only elected one official leader, Ueda, but he soon realized that he could not lead such a divided group of members and gave up on it. From that day on, the "unofficial" leader of the group was always Kame.
KAT-TUN was one of the very few groups that had two lead vocalists. Kame and Jin understood and supported each other's voices very well. That's why Jin's departure took a toll on KAT-TUN. But even after his departure, they still released many iconic songs. Honestly, if they had just put their egos aside and acted like mature men by accepting that they would never be as popular as Kame, they could still be in a good place as a group. Even before they debuted, Kame started to carry this group. The reason he even agreed to debut with Yamapi before debuting with his group was to give his group a debut momentum, and they hated him for that. And guess what? Nobuta broke rating records. Seishun Amigo broke single records and became an iconic song that is still sung today. Kame, with his baseball background, was a real team player, and I wish his group members could realize that. If the group even made it to their 19th anniversary, they owe it to Kame. No one else.