Here's just a PARTIAL list of actors and actresses who have played SUPPORTING roles in CBM's over the years, often to actors who were much less lknown than them:
Jeff Bridges, Gwyneth Paltrow, Terrence Howard, Don Cheadle, Mickey Rourke, Sir Michael Caine, Gary Oldman, Liam Neeson, Ken Watanabe, Cillian Murphy, Tom Wilkinson, Heath Ledger, Tom Hardy, Anne Hathaway, Marion Cotillard, Sam Rockwell, Sir Ben Kingsley, Guy Pearce, Rebecca Hall, Mark Strong, Michael Clarke Duncan, Geoffrey Rush, Russell Crowe, Amy Adams, Laurence Fishburne, Michael Shannon, Kevin Costner, Diane Lane, William Hurt, Tim Roth, Sir Anthony Hopkins, Samuel L. Jackson, Tommy Lee Jones, Stanley Tucci, Peter Dinklage, Josh Brolin, Oscar Isaacs, Mr. Eko from Lost, Andy Serkis, Hugo Weaving, ROBERT FREAKING REDFORD, and the list goes on and on and on and on.
So please, tell me again how taking on a supporting role (or villain) role is a "step down" for an actor again? Besides, Pine still gets to play the lead, or one of the three leads at least, in ST.