I think there comes a point for a team when you move a different direction. Not because a player will never get it, but because the window for him to get it with your team is closing and/or other options present themself.
At some point, if Miller doesn't take the next steps with his game, the Rangers will have to decide if they want to pursue a player who more readily contributes to their success and allows them to capitalize on their current window. This player may come from inside the organization, or he may come from outside the organization. Said player would, theoretically, receive and/or earn ice time that comes at the expense of players who didn't take advantage of their opportunity - which could include Miller.
The question wouldn't so much be whether Miller's NHL career is over, so much as whether the Rangers go with guys who potentially show them more - be it Fast, Etem, Lindberg or whoever. If that happens, it doesn't mean Miller is done as a Ranger, but it does take away some level of opportunity, if not ice time. That makes it that much harder to take those next steps.
The Rangers would also have to weigh the cost of resigning him, his potential value to other teams and whether they want to cut bait and try and recoup some return from a team that may be interested in Miller and capable of providing an opportunity.
In his upcoming age 22/23 season, and with several pro years of experience under his belt, Miller is expected to find a consistent niche on this team. He doesn't have to be the finished product, but he does need to win and keep a regular role. If he doesn't, the Rangers are likely to explore other options at his expense.