Solving the Roidmudes cases is the major center of the plot. And Shinnosuke is on it every step of the way, leading the charge. If your hypothetical Kuroko fought every Gedoushuu, up to and including Dokoku, solving the problem of their threat to humanity, just like Shinnosuke is doing, then yes, he would be the central character. And he would be extremely recognizable. That would make your hypothetical Takeru "useless", though.
Yes, solving the Roidmude cases take up most time every episode but it is not story of Drive. Except killing monster is all you need to call someone main character. Asumu and Hibiki say hi.
For me, even if Kuroko is the one who fight, Takeru is still one of main character. He is their master, the one who give order to kill Gedoshuu, his decision make story plot. Even though all he do when his retainer fight is just sit around. That is what a warlord usually do anyway.
See, this hypothetical Kuroko stuff is nonsense. If you're going to do analogies, make it a correct one.
Shinnosuke may lack background, but he's definitely not static nor lacking in depth. He experiences conflicts and is continually changing as the series goes on.
Of course it is nonsense. It will be such joke series if it is really happen.
Shinnosuke is not that terrible, he still have many possibilities await for him. But until now, he just forcedly blended to story, become anything they need him to do. "This type need passion, okay, I'm fired up". "This type need to be cool person, I'm so angry but relax I'm calm". "Wow a biker, by the way, I am car enthusiast". " You are a good boy but reckless, it's alright I will watch your back". And many thing happened, so I can just see him only as a handy good guy, good but not material of main character for me.
I wait him for his own story, where his own decision is the plot, not just goody-two-shoes who support everyone wish.
Right being optimistic toward the future is not unique. That's practically every Sentai Red AND practically every Kamen Rider in existence. Because you know, a hero that's pessimistic is such an utter downer.
Unique is not necessarily thing for principle. We have a pesimistic, a coward, a lazy ass, an arogant before but they strife to become better person. And friendly is not that unique anyway. Amazon say hi.