Well, somebody missed it. Maybe it was you. The speed, the lateral movement, the strength, the experience, none of that was there last year, because of the injury. All everybody heard last year was how good Smith looked but when he got on the field, all of that crumbled. He was not ready and there is no guarantee he will be this year either. You can't base your defense on that.
Hitchens understands the game and the scheme, he is a much better tackler, he is stronger, he can stack and shed, he can scrape, he is physically stronger, he fills the whole much better and he holds up better at the MLB position. He can also play multiple LB positions so he gives you something there as well.
The real problem here is that you are trying to fit a square peg into a round hole, with Smith at MLB IMO. He is not that guy.