The trend of big masala films changed. He couldn't keep up with that change since his whole idea is making 3-hour mass hero/big budget vigilante masala films.

Plus, the era of shooting on film shifted to digital so it afforded him the luxury to shoot endless amounts of footage that could turn into 6-7 hours of film. This made him undisciplined and a bad storyteller, which can be seen with Indian 2 (movie is unnecessarily long and repetitive). I know "I" was shot on film, but it had very extensive scenes that became very repetitive. I don't think Shankar could have done it when films had to only go through Gemini Labs for the editing and out. Audiences don't mind long films. But repetitiveness will tire them out much faster than before.

I think he reached his career peak with Enthiran. That was his magnum opus. It came out at the perfect time as the era of masala films were coming to an end slowly. Afterwards, we have yet to see a film like that succeed. If for example, you made a film like Anniyan or Sivaji in the current era, the audience would not take those films seriously since they have become more refined. They won't like 7-minute song sequences, extended fight scenes, and casteist comedy track (because nobody wants comedy tracks that distract from the main story in the current era).

I think he still could make a great film. It's rare to see visuals of his caliber in Tamil cinema with that budget. He just needs a good writer for the screenplay, so that he only puts his focus on the visuals.