Grown up Robins would just become reduced versions of Batman. The guy is world's greatest detective, top tier tactician/strategist, top 10 skilled fighters, top 10 billionaires, a master of stealth & escapology. How can any Robin become more well rounded than Batman? They simply can't. They work as Batman's sidekicks. Maybe they have different personalities or moral compass that can make them stand out since Batman is this dark, brooding no kill hero but you already have a cheerful hero like Green Arrow or an assassin with his own code of honor like Deathstroke.