Well humans aren't really physiologically designed to perform overhead movements well, and anything over 90 MPH is considered a near certainty to eventually cause an injury, so in a way we've already surpassed what's supposed to be "possible". I think it's just going to get faster and faster. The way humans are evolving- we're bigger, stronger, and we're learning so much more about training and development and whatnot. I think at some point, maybe not so far down the road, the majority of big league pitchers will be sitting in the mid-upper 90's.