Every director wants to and for the most part select suitable actors for the roles in his film. Sometimes there may be compromises for various reasons but not in the case of lead actors. There are times even if the lead actor suited the role and did well, for various reasons audience may not get connected with the actor's portrayal of the role. Or the audience might have seen the actor in similar roles multiple times. Hence the impact decreases. Just as it had happened with Maharshi. Though it's a film where Mahesh is virtually irreplaceable, his playing similar roles in his recent films impacted the BO of the film to an extent. We know Raja Sekhar was an asset for Kalki but Nagarjuna would have made it a bigger hit.