A different actor every time? So everyone can angst and whine about it every single bloody time? Seriously? Yeah, no thanks. I know I know I know, what's a forum for but to angst and whine, you say. Fine. *grumps* Just putting forth my tuppenny.
Also, I'm pretty sure it's the norm to operate under a trilogy 'contract' these days, ie, actors agreeing to return should there be more and so on. Read that somewhere, or saw an interview, or both. Might even have been here. Doesn't really guarantee sequels.
I was reading up about box office and finance etc on one of the comic sites and it's quite frightening how well a film has to do to get a sequel. Makes it even more amazing that there was a Serenity movie.