Everyone knows Mr Bajaj was originally meant to be an 8 week cameo appearance in the show. Things took an unexpected turn when the popularity of the character soared. RR was indeed phenomenal as Mr Bajaj and nobody could have done that part better than him. Surpassing the so-called male lead and rising to greater heights in an established show is no ordinary feat. But how many characters played by RR have been as popular and lasting as Mr Bajaj? One can't discredit the character while praising its face. It wasn't just RR's Rishabh Bajaj that surpassed sissy in terms of individual popularity, PRB did the same in 2004. So this parallel vs main lead debate hardly matters to us because the former is clearly ahead by a huge margin.
I haven't watched KSG's acting in his successful shows. But given the quality and cast and crew of this show, he doesn't have to as phenomenal as RR. With an average performance he can outperform them all.