Except Smith had been in the league 4 years and had a vet minimum salary that was almost double what you'd pay a back roster rookie replacement, compared to Brown who's only been in the league two seasons so far. Smith is exactly the case I was talking about above where it makes sense to let him walk if you think you can find a comparable player. Though in this case, they actually invested a fairly significant pick at RB in the draft in order to upgrade that spot.
Brown can get beat out, like any other bottom-of-the-roster player. But it will be for a guy who gives you something comparable on ST and, secondarily, on offense and not in favor of a guy who runs 'better' routes or has better hands. It'll be because you can get a comparable player for the snaps he'll actually play, for cheaper, elsewhere.