take your time, don't live too fast
You see this constantly in the NFL. Teams move on from veterans in hopes and expectations that the next guy in line will step up. That's one of the great things about the NFL. Lamar is being given this opportunity to showcase what he's got, and Miami is willing to give him that chance. He obviously showed something in year one that the staff liked, and that skill made Bush expendable (since it looks like he won't be back).
I think the biggest positive about Miller was his ability to move north and south while still making people miss. He didn't dance around. He got the ball and ran the damn thing.
I also think we can expect a lot from Jonas Gray. The team was so confident in Gray as a player that they signed him to a two-year deal in order to give him the entire 2012 season to rehab his knee and get his health back. I can't think of another time where a player was given that opportunity. He's not a guy a lot of people are counting on or talking about, but I really do think he has the potential to be a surprise when training camp opens.
With the dozen or so picks Miami has in the draft, one of those will probably be used on a running back. There's no reason not to select a running back in the later rounds.
I wouldn't call Miller a "lock to start," though given the attitude of the team towards Bush, all signs point to Miller being the guy in 2013.