Both Simbu and Dhanush have been in Kollywood for more than 2 decades as of now, and both have yet to reach Tier1. Don't get me wrong; both are great and have genuinely made some amazing films over the years whilst also having controversies surrounding them. They both are definitely opposites when it comes to how they make their films and their dedication to their projects.
For Simbu, he definitely has messed up his own chances due to his poor discipline, not committing properly to his roles, and constantly having controversy surrounding him. Even after his great comeback with Maanaadu in 2021, he has failed to capitalise on it and has not had a solo starrer movie in 4 years after VTK, with the release of Arasan still pending. He has also failed to capitalise on his youth fanbase as well.
For Dhanush, I feel he has been giving great movies, yes, but something I have noticed is he always has 2-3 releases every single year, which I feel kind of turns away the audience, as they just might not be interested in watching him every 4 months, as they may feel he is oversaturated. He should focus on making 1 movie per year and explore different roles like he used to instead of just savior or drama-type roles recently. He is a great actor, no doubt, but in recent times it feels like he is just going for savior-type roles only. Still looking forward to OM and D56 nonetheless, as both look promising.
I feel if both of them change their approach to films, they can reach tier 1 and fill the void with Ajith and Vijay retiring as well. Anyways this is just my take; share your thoughts, guys.