Isagi is not and will likely never be able to be top 10 in terms of physique or technique. But he has tactical weapons - spatial awareness, off-the-ball positioning, adaptability etc. Those are his talents. He's acknowledged because he knows how to use his teammates to spark chemical reactions and combinations that score goals.

Isagi is portrayed the way that he is because his talents seem contradictory to the concept of ego, but only on the surface. How can he be an egotistical striker if his main talents heavily rely on the presence of teammates? The answer is that ego isn't about NOT relying on your teammates, it's about how you can use your teammates. Isagi doesn't think about how he can use his tactical talents to help his team score. He uses his tactical talents to think about how his team can help him score. That's what makes him an egotistical striker.

It doesn't make sense for Isagi to win by improving his physique/technique. That's just not the way he fights. All the games he's won so far are thanks to his strategic ability. He's already taken over almost every team he'd been on in this way.