The action movie paradigm has changed a great deal in the last 20+ years. Adrien Brody has no need to fill the shoes of any actor or character. Especially Arnold Schwarzenegger. If Arnold Schwarzenegger was 35 years-old now in 2009, he would have a much tougher time getting acting gigs than he did in 1982. The super-pumped-up aesthetic of the 80s is over, thankfully. The modern male lead in an action film doesn't have to be, in and of itself, a spectacle. I've gone over much of this in a previous entry. If you'll like to read it, click here.
An actor's physique is definitely important. Woody Allen could never play Rambo. There has to be a degree of verisimilitude with an action lead. But that door swings both ways. Before Predator was shot, Schwarzenegger was actually too big. He was asked by the director to slim down to the body type of a typical Special Forces soldier. In fact, if you ever have the pleasure of meeting a typical Navy SEAL, you'll see that they look more like Topher Grace than Arnold Schwarzenegger. The military's training program doesn't focus on bodybuilding. There is a fair amount of running and obstacle negotiation. No way in hell a guy with Arnold's body could traverse a thick, mountainous jungle. He's just not built for it. Too much bulk. Special Forces operators are supposed to be small and lean.
As if actors weren't already pummeled with enough self-doubt and deprecation, the 80s added more with a crippling sense of male inferiority. Schwarzenegger and Stallone really changed the game–the body was now a separate entity from the actor. It had to be ripped to an unnatural extent. Whether this trend was a reflection of society or a creation from cinema and popular culture, I have no idea. But I do know that the paradigm has changed significantly.
And so, no, I don't have a problem with Adrien Brody taking on the lead in the new Predators film. And yes, I am a huge fan of the original Predator film and Arnold Schwarzenegger. I think Brody is a terrific actor and as such, his skills can absolutely support him in his role. I like to think that acting is enough for an actor–they don't need to be burdened with a huge set of biceps.